ms. wilson's math classes -...
TRANSCRIPT
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Determine the common ratio, and find the next three terms of each geometric sequence.
1.
SOLUTION: First, find the common ratio.
= –2
= –2
The common ratio is –2. Multiply the third term by –2 to find the fourth term, and so on. –1(–2) = 2 2(–2) = –4 –4(–2) = 8 Therefore, the next three terms are 2, –4, and 8.
2.
SOLUTION: First, find the common ratio.
The common ratio is . Multiply the third term
by to find the fourth term, and so on.
Therefore, the next three terms are – , ,
and – .
3. 0.5, 0.75, 1.125, …
SOLUTION: First, find the common ratio. 0.75 ÷ 0.5 = 1.5 1.125 ÷ 0.75 = 1.5 The common ratio is 1.5. Multiply the third term by 1.5 to find the fourth term, and so on. 1.125(1.5) = 1.6875 1.6875(1.5) = 2.53125 2.53125(1.5) = 3.796875 Therefore, the next three terms are 1.6875, 2.53125, and 3.796875.
4. 8, 20, 50, …
SOLUTION: First, find the common ratio. 20 ÷ 8 or 2.5 50 ÷ 20 or 2.5 The common ratio is 2.5. Multiply the third term by 2.5 to find the fourth term, and so on. 50(2.5) = 125 125(2.5) = 312.5 312.5(2.5) = 781.25 Therefore, the next three terms are 125, 312.5, and 781.25.
5. 2x, 10x, 50x, …
SOLUTION: First, find the common ratio. 10x ÷ 2x = 5 50x ÷ 10x = 5 The common ratio is 5. Multiply the third term by 5 to find the fourth term, and so on. 5(50x) = 250x 5(250x) =1250x 5(1250x) =6250x Therefore, the next three terms are 250x, 1250x, and 6250x.
6. 64x, 16x, 4x, …
SOLUTION: First, find the common ratio.
16x ÷ 64x =
4x ÷ 16x =
The common ratio is . Multiply the third term by
to find the fourth term, and so on.
Therefore, the next three terms are x, x, and
x.
7. x + 5, 3x + 15, 9x + 45, …
SOLUTION: First, find the common ratio.
The common ratio is 3. Multiply the third term by 3 to find the fourth term, and so on. 3(9x + 45) = 27x +135 3(27x +135) = 81x + 405 3(81x + 405) = 243x + 1215 Therefore, the next three terms are 27x +135, 81x + 405, and 243x + 1215.
8. –9 – y , 27 + 3y , –81 – 9y , …
SOLUTION: First, find the common ratio.
The common ratio is –3. Multiply the third term by –3 to find the fourth term, and so on. –3(–81 – 9y) = 243 + 27y –3(243 + 27y) = –729 – 81y –3(–729 – 81y) = 2187 + 243y Therefore, the next three terms are 243 + 27y , –729 – 81y , and 2187 + 243y .
9. GEOMETRY Consider a sequence of circles withdiameters that form a geometric sequence: d1, d2,
d3, d4, d5.
a. Show that the sequence of circumferences of the circles is also geometric. Identify r. b. Show that the sequence of areas of the circles isalso geometric. Identify the common ratio.
SOLUTION: a. Sample answer: The circumference of a circle isgiven by C = πd. So, the sequence of circumferences of the circles is πd1, πd2, πd3, πd4,
πd5.
Find the common ratio.
b. Sample answer: The area of a circle is given by
C = πr2 or So, the sequence of areas
of the circles is
Find the common ratio.
Write an explicit formula and a recursive formula for finding the nth term of each geometric sequence.
10. 36, 12, 4, …
SOLUTION: First, find the common ratio.
12 ÷ 36 =
4 ÷ 12 =
For an explicit formula, substitute a1 = 36 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 36, an =
11. 64, 16, 4, …
SOLUTION: First, find the common ratio.
16 ÷ 64 =
4 ÷ 16 =
For an explicit formula, substitute a1 = 64 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 64, an =
12. −2, 10, −50, …
SOLUTION: First, find the common ratio. 10 ÷ –2 = –5 –50 ÷ 10 = –5 For an explicit formula, substitute a1 = –2 and r = –
5 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = –2, an =
13. 4, −12, 36, …
SOLUTION: First, find the common ratio. –12 ÷ 4 = –3 36 ÷ –12 = –3 For an explicit formula, substitute a1 = 4 and r = –3
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 4, an =
14. 4, 8, 16, …
SOLUTION: First, find the common ratio. 8 ÷ 4 = 2 16 ÷ 8 = 2 For an explicit formula, substitute a1 = 4 and r = 2
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 4, an =
15. 20, 30, 45, …
SOLUTION: First, find the common ratio. 30 ÷ 20 = 1.5 45 ÷ 30 = 1.5 For an explicit formula, substitute a1 = 20 and r =
1.5 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 20, an =
16. 15, 5, , …
SOLUTION: First, find the common ratio.
5 ÷ 15 =
÷ 5 =
For an explicit formula, substitute a1 = 15 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 15, an =
17. , , , …
SOLUTION: First, find the common ratio.
÷ = 2
÷ = 2
For an explicit formula, substitute a1 = and r =
2 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = , an =
18. CHAIN E-MAIL Melina receives a chain e-mail that she forwards to 7 of her friends. Each of her friends forwards it to 7 of their friends.
a. Write an explicit formula for the pattern. b. How many will receive the e-mail after 6 forwards?
SOLUTION: Melina receives a chain e-mail, forwards it to 7 friends, and each friend forwards it to 7 friends.
Therefore, a1 = 1, a2 = 7, and a3 = 49. The
common ratio is 7. For an explicit formula, substitute a1 = 1 and r = 7
in the nth term formula.
b. Use the explicit formula you found in part a to find a6.
Therefore, after 6 forwards 16,807 people will havereceived the e-mail.
19. BIOLOGY A certain bacteria divides every 15 minutes to produce two complete bacteria. a. If an initial colony contains a population of b0
bacteria, write an equation that will determine the
number of bacteria bt present after t hours.
b. Suppose a Petri dish contains 12 bacteria. Use the equation found in part a to determine the number of bacteria present 4 hours later.
SOLUTION: a. Initially, there is 1 bacterium. After 15 minutes, there will be 2 bacteria, after 30 minutes there will be 4 bacteria, after 45 minutes there will be 8 bacteria, and after 1 hour there will be 16 bacteria.
So, in terms of hours, b0 = 1 and b1 = 16. Find the
common ratio.
16 ÷ 1 = 16 Write an explicit formula using r = 16.
b. Substitute b0 = 12 and t = 4 into the equation you
found in part a.
Find the specified term for each geometric sequence or sequence with the given characteristics.
20. a9 for 60, 30, 15, …
SOLUTION: First, find the common ratio.
30 ÷ 60 =
15 ÷ 30 =
Use the formula for the nth term of a geometric
sequence to find a9.
21. a4 for 7, 14, 28, …
SOLUTION: First, find the common ratio. 14 ÷ 7 = 2 28 ÷ 14 = 2 Use the formula for the nth term of a geometric
sequence to find a4.
22. a5 for 3, 1, , …
SOLUTION: First, find the common ratio.
1 ÷ 3 =
÷ 1 =
Use the formula for the nth term of a geometric
sequence to find a5.
23. a6 for 540, 90, 15, …
SOLUTION: First, find the common ratio.
90 ÷ 540 =
15 ÷ 90 =
Use the formula for the nth term of a geometric
sequence to find a6.
24. a7 if a3 = 24 and r = 0.5
SOLUTION:
Use the values of a3 and r to find a2.
Next, use the values of a2 and r to find a1.
Use the formula for the nth term of a geometric
sequence to find a7.
Another method would be to consider that the 7th term is 4 terms from the 3rd term. Therefore,
multiply the 3rd term by r4.
25. a6 if a3 = 32 and r = –0.5
SOLUTION:
Use the values of a3 and r to find a2.
Next, use the values of a2 and r to find a1.
Use the formula for the nth term of a geometric
sequence to find a6.
Another method would be to consider that the 6th term is 3 terms from the 3rd term. Therefore,
multiply the 3rd term by r3.
26. a6 if a1 = 16,807 and r =
SOLUTION: Use the formula for the nth term of a geometric
sequence to find a6.
27. a8 if a1 = 4096 and r =
SOLUTION: Use the formula for the nth term of a geometric
sequence to find a8.
28. ACCOUNTING Julian Rockman is an accountantfor a small company. On January 1, 2009, the company purchased $50,000 worth of computers, printers, scanners, and hardware. Because this equipment is a company asset, Mr. Rockman needsto determine how much the computer equipment is presently worth. He estimates that the computer equipment depreciates at a rate of 45% per year. What value should Mr. Rockman assign the equipment in his 2014 year-end accounting report?
SOLUTION:
The equipment is originally worth $50,000, so a1 =
50,000. Because the equipment depreciates at a rate of 45% per year, the value of the equipment ona given year will be 100% – 45% or 55% of the value the previous year. So, r = 0.55. The first term
a1 corresponds to the year 2009, so the year 2014
corresponds to a6.
Use the formula for the nth term of a geometric
sequence to find the a6.
Therefore, the value of the equipment in 2014 is about $2516.42.
29. Find the sixth term of a geometric sequence with a first term of 9 and a common ratio of 2.
SOLUTION: Use the formula for the nth term of a geometric
sequence to find the a6.
30. If r = 4 and a8 = 100, what is the first term of the geometric sequence?
SOLUTION:
Substitute a8 = 100, r = 4, and n = 8 into the
formula for the nth term of a geometric sequence
to find the a1.
31. X GAMES Refer to the beginning of the lesson. The X Games netted approximately $40 million in revenue in 2002. If the X Games continue to generate 13% more revenue each year, how much revenue will the X Games generate in 2020?
SOLUTION: The X Games netted about $40,000,000 in 2002, so
a1 = $40,000,000. Because the games generate
13% or 0.13 more revenue each year, the amount of revenue generated on a given year will be 1.13 times the revenue from the previous year. So, r =
1.13. The first term a1 corresponds to the year
2002, so the year 2020 corresponds to a19.
Use the formula for the nth term of a geometric
sequence to find the a19.
Therefore, the X Games will generate about $360.97 million in 2020.
Find the indicated geometric means for each pair of nonconsecutive terms.
32. 4 and 256; 2 means
SOLUTION: The sequence will resemble 4, __?__ , __?__ , 256. Note that a1 = 4, n = 4, and a4 = 256. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is 4. Use r to find the geometric means. 4(4) = 16 16(4) = 64 Therefore, a sequence with two geometric means between 4 and 256 is 4, 16, 64, 256.
33. 256 and 81; 3 means
SOLUTION: The sequence will resemble 256, __?__ , __?__ ,
__?__ , 81. Note that a1 = 256, n = 5, and a5 = 81.
Find the common ratio using the nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
r =
r =
Therefore, a sequence with three geometric means between 256 and 81 is 256, –192, 144, –108, 81 or 256, 192, 144, 108, 81.
34. and 7; 1 mean
SOLUTION:
The sequence will resemble , __?__ , 7. Note
that a1 = , n = 3, and a3 = 7. Find the common
ratio using the nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
Therefore, a sequence with one geometric mean
between and 7 is , –2, 7 or , 2, 7.
35. –2 and 54; 2 means
SOLUTION: The sequence will resemble –2, __?__ , __?__ , 54. Note that a1 = –2, n = 4, and a4 = 54. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is –3. Use r to find the geometric means. –2(–3) = 6 6(–3) = –18 Therefore, a sequence with two geometric means between –2 and 54 is –2, 6, –18, 54.
36. 1 and 27; 2 means
SOLUTION: The sequence will resemble 1, __?__ , __?__ , 27. Note that a1 = 1, n = 4, and a4 = 27. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is 3. Use r to find the geometric means. 1(3) = 3 3(3) = 9 Therefore, a sequence with two geometric means between 1 and 27 is 1, 3, 9, 27.
37. 48 and −750; 2 means
SOLUTION: The sequence will resemble 48, __?__ , __?__ , –750. Note that a1 = 48, n = 4, and a4 = 48. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
= –120
= 300
Therefore, a sequence with two geometric means between 48 and –750 is 48, –120, 300, –750.
38. i and −1; 4 means
SOLUTION: The sequence will resemble i, __?__ , __?__ , __?__ , __?__ , –1. Note that a1 = i, n = 6, and a6 = –1. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is i. Use r to find the geometric means. i(i) = –1 –1(i) = –i –i(i) = 1 1(i) = i Therefore, a sequence with two geometric means
between i and −1 is i, –1, –i, 1, i, –1.
39. t 8 and t – 7; 4 means
SOLUTION:
The sequence will resemble t8, __?__ , __?__ ,
__?__ , __?__ , t – 7
.
Note that a1 = t8, n = 6, and a6 = t
– 7. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
= t5
= t2
= t–1
= t–4
Therefore, a sequence with two geometric means
between t8 and t
– 7 is t
8, t
5, t
2, t–1
, t–4
, t – 7
.
Find the sum of each geometric series described.
40. first six terms of 3 + 9 + 27 + …
SOLUTION: First, find the common ratio. 9 ÷ 3 = 3 27 ÷ 9 = 3 The common ratio is 3. Use Formula 1 for the sum of a finite geometric series.
41. first nine terms of 0.5 + (–1) + 2 + …
SOLUTION: First, find the common ratio. –1 ÷ 0.5 or –2 2 ÷ –1 or –2 The common ratio is –2. Use Formula 1 for the sum of a finite geometric series.
42. first eight terms of 2 + 2 + 6 + …
SOLUTION: First, find the common ratio.
2 ÷ 2 or
6 ÷ 2 or
The common ratio is . Use Formula 1 for the sum of a finite geometric series.
43. first n terms of a1 = 4, an = 2000, r = −3
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
44. [Contains update not in print edition] first n terms of a1 = 5, an = 1,310,720, r = 4
SOLUTION: [Solution for updated problem] Use Formula 2 for the nth partial sum of a geometric series.
45. first n terms of a1 = 3, an = 46,875, r = −5
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
46. first n terms of a1 = −8, an = −256, r = 2
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
47. first n terms of a1 = −36, an = 972, r = 7
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
Find each sum.
48.
SOLUTION:
Find n, a1, and r.
Substitute n = 6, a1 = 5, and r = 2 into the formula
for the sum of a finite geometric series.
49.
SOLUTION:
Find n, a1, and r.
Substitute n = 5, a1 = −4, and r = 3 into the formula
for the sum of a finite geometric series.
50.
SOLUTION:
Find n, a1, and r.
Substitute n = 5, a1 = 1, and r = −3 into the formula
for the sum of a finite geometric series.
51.
SOLUTION:
Find n, a1, and r.
Substitute n = 6, a1 = 2, and r = 1.4 into the
formula for the sum of a finite geometric series.
52.
SOLUTION:
Find n, a1, and r.
Substitute n = 6, a1 = 100, and r = into the
formula for the sum of a finite geometric series.
53.
SOLUTION:
Find n, a1, and r.
Substitute n = 9, a1 = , and r = −3 into the
formula for the sum of a finite geometric series.
54.
SOLUTION:
Find n, a1, and r.
Substitute n = 7, a1 = 144, and r = into the
formula for the sum of a finite geometric series.
55.
SOLUTION:
Find n, a1, and r.
Substitute n = 20, a1 = 3, and r = 2 into the formula
for the sum of a finite geometric series.
If possible, find the sum of each infinite geometric series.
56. + + + …
SOLUTION: First, find the common ratio.
÷ =
÷ =
The common ratio r is 1. Therefore, this infinite geometric series has no sum.
58. 18 + (–27) + 40.5 + ...
SOLUTION: First, find the common ratio. –27 ÷ 18 = –1.5 40.5 ÷ –27 = –1.5 The common ratio r is > 1. Therefore, this infinite geometric series has no sum.
59. 12 + (–7.2) + 4.32 + ...
SOLUTION: First, find the common ratio. –7.2 ÷ 12 = –0.6 4.32 ÷ –7.2 = –0.6 The common ratio r is 1. Therefore, this infinite geometric series has no sum.
59. 12 + (–7.2) + 4.32 + ...
SOLUTION: First, find the common ratio. –7.2 ÷ 12 = –0.6 4.32 ÷ –7.2 = –0.6 The common ratio r is
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Determine the common ratio, and find the next three terms of each geometric sequence.
1.
SOLUTION: First, find the common ratio.
= –2
= –2
The common ratio is –2. Multiply the third term by –2 to find the fourth term, and so on. –1(–2) = 2 2(–2) = –4 –4(–2) = 8 Therefore, the next three terms are 2, –4, and 8.
2.
SOLUTION: First, find the common ratio.
The common ratio is . Multiply the third term
by to find the fourth term, and so on.
Therefore, the next three terms are – , ,
and – .
3. 0.5, 0.75, 1.125, …
SOLUTION: First, find the common ratio. 0.75 ÷ 0.5 = 1.5 1.125 ÷ 0.75 = 1.5 The common ratio is 1.5. Multiply the third term by 1.5 to find the fourth term, and so on. 1.125(1.5) = 1.6875 1.6875(1.5) = 2.53125 2.53125(1.5) = 3.796875 Therefore, the next three terms are 1.6875, 2.53125, and 3.796875.
4. 8, 20, 50, …
SOLUTION: First, find the common ratio. 20 ÷ 8 or 2.5 50 ÷ 20 or 2.5 The common ratio is 2.5. Multiply the third term by 2.5 to find the fourth term, and so on. 50(2.5) = 125 125(2.5) = 312.5 312.5(2.5) = 781.25 Therefore, the next three terms are 125, 312.5, and 781.25.
5. 2x, 10x, 50x, …
SOLUTION: First, find the common ratio. 10x ÷ 2x = 5 50x ÷ 10x = 5 The common ratio is 5. Multiply the third term by 5 to find the fourth term, and so on. 5(50x) = 250x 5(250x) =1250x 5(1250x) =6250x Therefore, the next three terms are 250x, 1250x, and 6250x.
6. 64x, 16x, 4x, …
SOLUTION: First, find the common ratio.
16x ÷ 64x =
4x ÷ 16x =
The common ratio is . Multiply the third term by
to find the fourth term, and so on.
Therefore, the next three terms are x, x, and
x.
7. x + 5, 3x + 15, 9x + 45, …
SOLUTION: First, find the common ratio.
The common ratio is 3. Multiply the third term by 3 to find the fourth term, and so on. 3(9x + 45) = 27x +135 3(27x +135) = 81x + 405 3(81x + 405) = 243x + 1215 Therefore, the next three terms are 27x +135, 81x + 405, and 243x + 1215.
8. –9 – y , 27 + 3y , –81 – 9y , …
SOLUTION: First, find the common ratio.
The common ratio is –3. Multiply the third term by –3 to find the fourth term, and so on. –3(–81 – 9y) = 243 + 27y –3(243 + 27y) = –729 – 81y –3(–729 – 81y) = 2187 + 243y Therefore, the next three terms are 243 + 27y , –729 – 81y , and 2187 + 243y .
9. GEOMETRY Consider a sequence of circles withdiameters that form a geometric sequence: d1, d2,
d3, d4, d5.
a. Show that the sequence of circumferences of the circles is also geometric. Identify r. b. Show that the sequence of areas of the circles isalso geometric. Identify the common ratio.
SOLUTION: a. Sample answer: The circumference of a circle isgiven by C = πd. So, the sequence of circumferences of the circles is πd1, πd2, πd3, πd4,
πd5.
Find the common ratio.
b. Sample answer: The area of a circle is given by
C = πr2 or So, the sequence of areas
of the circles is
Find the common ratio.
Write an explicit formula and a recursive formula for finding the nth term of each geometric sequence.
10. 36, 12, 4, …
SOLUTION: First, find the common ratio.
12 ÷ 36 =
4 ÷ 12 =
For an explicit formula, substitute a1 = 36 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 36, an =
11. 64, 16, 4, …
SOLUTION: First, find the common ratio.
16 ÷ 64 =
4 ÷ 16 =
For an explicit formula, substitute a1 = 64 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 64, an =
12. −2, 10, −50, …
SOLUTION: First, find the common ratio. 10 ÷ –2 = –5 –50 ÷ 10 = –5 For an explicit formula, substitute a1 = –2 and r = –
5 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = –2, an =
13. 4, −12, 36, …
SOLUTION: First, find the common ratio. –12 ÷ 4 = –3 36 ÷ –12 = –3 For an explicit formula, substitute a1 = 4 and r = –3
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 4, an =
14. 4, 8, 16, …
SOLUTION: First, find the common ratio. 8 ÷ 4 = 2 16 ÷ 8 = 2 For an explicit formula, substitute a1 = 4 and r = 2
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 4, an =
15. 20, 30, 45, …
SOLUTION: First, find the common ratio. 30 ÷ 20 = 1.5 45 ÷ 30 = 1.5 For an explicit formula, substitute a1 = 20 and r =
1.5 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 20, an =
16. 15, 5, , …
SOLUTION: First, find the common ratio.
5 ÷ 15 =
÷ 5 =
For an explicit formula, substitute a1 = 15 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 15, an =
17. , , , …
SOLUTION: First, find the common ratio.
÷ = 2
÷ = 2
For an explicit formula, substitute a1 = and r =
2 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = , an =
18. CHAIN E-MAIL Melina receives a chain e-mail that she forwards to 7 of her friends. Each of her friends forwards it to 7 of their friends.
a. Write an explicit formula for the pattern. b. How many will receive the e-mail after 6 forwards?
SOLUTION: Melina receives a chain e-mail, forwards it to 7 friends, and each friend forwards it to 7 friends.
Therefore, a1 = 1, a2 = 7, and a3 = 49. The
common ratio is 7. For an explicit formula, substitute a1 = 1 and r = 7
in the nth term formula.
b. Use the explicit formula you found in part a to find a6.
Therefore, after 6 forwards 16,807 people will havereceived the e-mail.
19. BIOLOGY A certain bacteria divides every 15 minutes to produce two complete bacteria. a. If an initial colony contains a population of b0
bacteria, write an equation that will determine the
number of bacteria bt present after t hours.
b. Suppose a Petri dish contains 12 bacteria. Use the equation found in part a to determine the number of bacteria present 4 hours later.
SOLUTION: a. Initially, there is 1 bacterium. After 15 minutes, there will be 2 bacteria, after 30 minutes there will be 4 bacteria, after 45 minutes there will be 8 bacteria, and after 1 hour there will be 16 bacteria.
So, in terms of hours, b0 = 1 and b1 = 16. Find the
common ratio.
16 ÷ 1 = 16 Write an explicit formula using r = 16.
b. Substitute b0 = 12 and t = 4 into the equation you
found in part a.
Find the specified term for each geometric sequence or sequence with the given characteristics.
20. a9 for 60, 30, 15, …
SOLUTION: First, find the common ratio.
30 ÷ 60 =
15 ÷ 30 =
Use the formula for the nth term of a geometric
sequence to find a9.
21. a4 for 7, 14, 28, …
SOLUTION: First, find the common ratio. 14 ÷ 7 = 2 28 ÷ 14 = 2 Use the formula for the nth term of a geometric
sequence to find a4.
22. a5 for 3, 1, , …
SOLUTION: First, find the common ratio.
1 ÷ 3 =
÷ 1 =
Use the formula for the nth term of a geometric
sequence to find a5.
23. a6 for 540, 90, 15, …
SOLUTION: First, find the common ratio.
90 ÷ 540 =
15 ÷ 90 =
Use the formula for the nth term of a geometric
sequence to find a6.
24. a7 if a3 = 24 and r = 0.5
SOLUTION:
Use the values of a3 and r to find a2.
Next, use the values of a2 and r to find a1.
Use the formula for the nth term of a geometric
sequence to find a7.
Another method would be to consider that the 7th term is 4 terms from the 3rd term. Therefore,
multiply the 3rd term by r4.
25. a6 if a3 = 32 and r = –0.5
SOLUTION:
Use the values of a3 and r to find a2.
Next, use the values of a2 and r to find a1.
Use the formula for the nth term of a geometric
sequence to find a6.
Another method would be to consider that the 6th term is 3 terms from the 3rd term. Therefore,
multiply the 3rd term by r3.
26. a6 if a1 = 16,807 and r =
SOLUTION: Use the formula for the nth term of a geometric
sequence to find a6.
27. a8 if a1 = 4096 and r =
SOLUTION: Use the formula for the nth term of a geometric
sequence to find a8.
28. ACCOUNTING Julian Rockman is an accountantfor a small company. On January 1, 2009, the company purchased $50,000 worth of computers, printers, scanners, and hardware. Because this equipment is a company asset, Mr. Rockman needsto determine how much the computer equipment is presently worth. He estimates that the computer equipment depreciates at a rate of 45% per year. What value should Mr. Rockman assign the equipment in his 2014 year-end accounting report?
SOLUTION:
The equipment is originally worth $50,000, so a1 =
50,000. Because the equipment depreciates at a rate of 45% per year, the value of the equipment ona given year will be 100% – 45% or 55% of the value the previous year. So, r = 0.55. The first term
a1 corresponds to the year 2009, so the year 2014
corresponds to a6.
Use the formula for the nth term of a geometric
sequence to find the a6.
Therefore, the value of the equipment in 2014 is about $2516.42.
29. Find the sixth term of a geometric sequence with a first term of 9 and a common ratio of 2.
SOLUTION: Use the formula for the nth term of a geometric
sequence to find the a6.
30. If r = 4 and a8 = 100, what is the first term of the geometric sequence?
SOLUTION:
Substitute a8 = 100, r = 4, and n = 8 into the
formula for the nth term of a geometric sequence
to find the a1.
31. X GAMES Refer to the beginning of the lesson. The X Games netted approximately $40 million in revenue in 2002. If the X Games continue to generate 13% more revenue each year, how much revenue will the X Games generate in 2020?
SOLUTION: The X Games netted about $40,000,000 in 2002, so
a1 = $40,000,000. Because the games generate
13% or 0.13 more revenue each year, the amount of revenue generated on a given year will be 1.13 times the revenue from the previous year. So, r =
1.13. The first term a1 corresponds to the year
2002, so the year 2020 corresponds to a19.
Use the formula for the nth term of a geometric
sequence to find the a19.
Therefore, the X Games will generate about $360.97 million in 2020.
Find the indicated geometric means for each pair of nonconsecutive terms.
32. 4 and 256; 2 means
SOLUTION: The sequence will resemble 4, __?__ , __?__ , 256. Note that a1 = 4, n = 4, and a4 = 256. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is 4. Use r to find the geometric means. 4(4) = 16 16(4) = 64 Therefore, a sequence with two geometric means between 4 and 256 is 4, 16, 64, 256.
33. 256 and 81; 3 means
SOLUTION: The sequence will resemble 256, __?__ , __?__ ,
__?__ , 81. Note that a1 = 256, n = 5, and a5 = 81.
Find the common ratio using the nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
r =
r =
Therefore, a sequence with three geometric means between 256 and 81 is 256, –192, 144, –108, 81 or 256, 192, 144, 108, 81.
34. and 7; 1 mean
SOLUTION:
The sequence will resemble , __?__ , 7. Note
that a1 = , n = 3, and a3 = 7. Find the common
ratio using the nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
Therefore, a sequence with one geometric mean
between and 7 is , –2, 7 or , 2, 7.
35. –2 and 54; 2 means
SOLUTION: The sequence will resemble –2, __?__ , __?__ , 54. Note that a1 = –2, n = 4, and a4 = 54. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is –3. Use r to find the geometric means. –2(–3) = 6 6(–3) = –18 Therefore, a sequence with two geometric means between –2 and 54 is –2, 6, –18, 54.
36. 1 and 27; 2 means
SOLUTION: The sequence will resemble 1, __?__ , __?__ , 27. Note that a1 = 1, n = 4, and a4 = 27. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is 3. Use r to find the geometric means. 1(3) = 3 3(3) = 9 Therefore, a sequence with two geometric means between 1 and 27 is 1, 3, 9, 27.
37. 48 and −750; 2 means
SOLUTION: The sequence will resemble 48, __?__ , __?__ , –750. Note that a1 = 48, n = 4, and a4 = 48. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
= –120
= 300
Therefore, a sequence with two geometric means between 48 and –750 is 48, –120, 300, –750.
38. i and −1; 4 means
SOLUTION: The sequence will resemble i, __?__ , __?__ , __?__ , __?__ , –1. Note that a1 = i, n = 6, and a6 = –1. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is i. Use r to find the geometric means. i(i) = –1 –1(i) = –i –i(i) = 1 1(i) = i Therefore, a sequence with two geometric means
between i and −1 is i, –1, –i, 1, i, –1.
39. t 8 and t – 7; 4 means
SOLUTION:
The sequence will resemble t8, __?__ , __?__ ,
__?__ , __?__ , t – 7
.
Note that a1 = t8, n = 6, and a6 = t
– 7. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
= t5
= t2
= t–1
= t–4
Therefore, a sequence with two geometric means
between t8 and t
– 7 is t
8, t
5, t
2, t–1
, t–4
, t – 7
.
Find the sum of each geometric series described.
40. first six terms of 3 + 9 + 27 + …
SOLUTION: First, find the common ratio. 9 ÷ 3 = 3 27 ÷ 9 = 3 The common ratio is 3. Use Formula 1 for the sum of a finite geometric series.
41. first nine terms of 0.5 + (–1) + 2 + …
SOLUTION: First, find the common ratio. –1 ÷ 0.5 or –2 2 ÷ –1 or –2 The common ratio is –2. Use Formula 1 for the sum of a finite geometric series.
42. first eight terms of 2 + 2 + 6 + …
SOLUTION: First, find the common ratio.
2 ÷ 2 or
6 ÷ 2 or
The common ratio is . Use Formula 1 for the sum of a finite geometric series.
43. first n terms of a1 = 4, an = 2000, r = −3
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
44. [Contains update not in print edition] first n terms of a1 = 5, an = 1,310,720, r = 4
SOLUTION: [Solution for updated problem] Use Formula 2 for the nth partial sum of a geometric series.
45. first n terms of a1 = 3, an = 46,875, r = −5
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
46. first n terms of a1 = −8, an = −256, r = 2
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
47. first n terms of a1 = −36, an = 972, r = 7
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
Find each sum.
48.
SOLUTION:
Find n, a1, and r.
Substitute n = 6, a1 = 5, and r = 2 into the formula
for the sum of a finite geometric series.
49.
SOLUTION:
Find n, a1, and r.
Substitute n = 5, a1 = −4, and r = 3 into the formula
for the sum of a finite geometric series.
50.
SOLUTION:
Find n, a1, and r.
Substitute n = 5, a1 = 1, and r = −3 into the formula
for the sum of a finite geometric series.
51.
SOLUTION:
Find n, a1, and r.
Substitute n = 6, a1 = 2, and r = 1.4 into the
formula for the sum of a finite geometric series.
52.
SOLUTION:
Find n, a1, and r.
Substitute n = 6, a1 = 100, and r = into the
formula for the sum of a finite geometric series.
53.
SOLUTION:
Find n, a1, and r.
Substitute n = 9, a1 = , and r = −3 into the
formula for the sum of a finite geometric series.
54.
SOLUTION:
Find n, a1, and r.
Substitute n = 7, a1 = 144, and r = into the
formula for the sum of a finite geometric series.
55.
SOLUTION:
Find n, a1, and r.
Substitute n = 20, a1 = 3, and r = 2 into the formula
for the sum of a finite geometric series.
If possible, find the sum of each infinite geometric series.
56. + + + …
SOLUTION: First, find the common ratio.
÷ =
÷ =
The common ratio r is 1. Therefore, this infinite geometric series has no sum.
58. 18 + (–27) + 40.5 + ...
SOLUTION: First, find the common ratio. –27 ÷ 18 = –1.5 40.5 ÷ –27 = –1.5 The common ratio r is > 1. Therefore, this infinite geometric series has no sum.
59. 12 + (–7.2) + 4.32 + ...
SOLUTION: First, find the common ratio. –7.2 ÷ 12 = –0.6 4.32 ÷ –7.2 = –0.6 The common ratio r is 1. Therefore, this infinite geometric series has no sum.
59. 12 + (–7.2) + 4.32 + ...
SOLUTION: First, find the common ratio. –7.2 ÷ 12 = –0.6 4.32 ÷ –7.2 = –0.6 The common ratio r is
-
Determine the common ratio, and find the next three terms of each geometric sequence.
1.
SOLUTION: First, find the common ratio.
= –2
= –2
The common ratio is –2. Multiply the third term by –2 to find the fourth term, and so on. –1(–2) = 2 2(–2) = –4 –4(–2) = 8 Therefore, the next three terms are 2, –4, and 8.
2.
SOLUTION: First, find the common ratio.
The common ratio is . Multiply the third term
by to find the fourth term, and so on.
Therefore, the next three terms are – , ,
and – .
3. 0.5, 0.75, 1.125, …
SOLUTION: First, find the common ratio. 0.75 ÷ 0.5 = 1.5 1.125 ÷ 0.75 = 1.5 The common ratio is 1.5. Multiply the third term by 1.5 to find the fourth term, and so on. 1.125(1.5) = 1.6875 1.6875(1.5) = 2.53125 2.53125(1.5) = 3.796875 Therefore, the next three terms are 1.6875, 2.53125, and 3.796875.
4. 8, 20, 50, …
SOLUTION: First, find the common ratio. 20 ÷ 8 or 2.5 50 ÷ 20 or 2.5 The common ratio is 2.5. Multiply the third term by 2.5 to find the fourth term, and so on. 50(2.5) = 125 125(2.5) = 312.5 312.5(2.5) = 781.25 Therefore, the next three terms are 125, 312.5, and 781.25.
5. 2x, 10x, 50x, …
SOLUTION: First, find the common ratio. 10x ÷ 2x = 5 50x ÷ 10x = 5 The common ratio is 5. Multiply the third term by 5 to find the fourth term, and so on. 5(50x) = 250x 5(250x) =1250x 5(1250x) =6250x Therefore, the next three terms are 250x, 1250x, and 6250x.
6. 64x, 16x, 4x, …
SOLUTION: First, find the common ratio.
16x ÷ 64x =
4x ÷ 16x =
The common ratio is . Multiply the third term by
to find the fourth term, and so on.
Therefore, the next three terms are x, x, and
x.
7. x + 5, 3x + 15, 9x + 45, …
SOLUTION: First, find the common ratio.
The common ratio is 3. Multiply the third term by 3 to find the fourth term, and so on. 3(9x + 45) = 27x +135 3(27x +135) = 81x + 405 3(81x + 405) = 243x + 1215 Therefore, the next three terms are 27x +135, 81x + 405, and 243x + 1215.
8. –9 – y , 27 + 3y , –81 – 9y , …
SOLUTION: First, find the common ratio.
The common ratio is –3. Multiply the third term by –3 to find the fourth term, and so on. –3(–81 – 9y) = 243 + 27y –3(243 + 27y) = –729 – 81y –3(–729 – 81y) = 2187 + 243y Therefore, the next three terms are 243 + 27y , –729 – 81y , and 2187 + 243y .
9. GEOMETRY Consider a sequence of circles withdiameters that form a geometric sequence: d1, d2,
d3, d4, d5.
a. Show that the sequence of circumferences of the circles is also geometric. Identify r. b. Show that the sequence of areas of the circles isalso geometric. Identify the common ratio.
SOLUTION: a. Sample answer: The circumference of a circle isgiven by C = πd. So, the sequence of circumferences of the circles is πd1, πd2, πd3, πd4,
πd5.
Find the common ratio.
b. Sample answer: The area of a circle is given by
C = πr2 or So, the sequence of areas
of the circles is
Find the common ratio.
Write an explicit formula and a recursive formula for finding the nth term of each geometric sequence.
10. 36, 12, 4, …
SOLUTION: First, find the common ratio.
12 ÷ 36 =
4 ÷ 12 =
For an explicit formula, substitute a1 = 36 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 36, an =
11. 64, 16, 4, …
SOLUTION: First, find the common ratio.
16 ÷ 64 =
4 ÷ 16 =
For an explicit formula, substitute a1 = 64 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 64, an =
12. −2, 10, −50, …
SOLUTION: First, find the common ratio. 10 ÷ –2 = –5 –50 ÷ 10 = –5 For an explicit formula, substitute a1 = –2 and r = –
5 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = –2, an =
13. 4, −12, 36, …
SOLUTION: First, find the common ratio. –12 ÷ 4 = –3 36 ÷ –12 = –3 For an explicit formula, substitute a1 = 4 and r = –3
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 4, an =
14. 4, 8, 16, …
SOLUTION: First, find the common ratio. 8 ÷ 4 = 2 16 ÷ 8 = 2 For an explicit formula, substitute a1 = 4 and r = 2
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 4, an =
15. 20, 30, 45, …
SOLUTION: First, find the common ratio. 30 ÷ 20 = 1.5 45 ÷ 30 = 1.5 For an explicit formula, substitute a1 = 20 and r =
1.5 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 20, an =
16. 15, 5, , …
SOLUTION: First, find the common ratio.
5 ÷ 15 =
÷ 5 =
For an explicit formula, substitute a1 = 15 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 15, an =
17. , , , …
SOLUTION: First, find the common ratio.
÷ = 2
÷ = 2
For an explicit formula, substitute a1 = and r =
2 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = , an =
18. CHAIN E-MAIL Melina receives a chain e-mail that she forwards to 7 of her friends. Each of her friends forwards it to 7 of their friends.
a. Write an explicit formula for the pattern. b. How many will receive the e-mail after 6 forwards?
SOLUTION: Melina receives a chain e-mail, forwards it to 7 friends, and each friend forwards it to 7 friends.
Therefore, a1 = 1, a2 = 7, and a3 = 49. The
common ratio is 7. For an explicit formula, substitute a1 = 1 and r = 7
in the nth term formula.
b. Use the explicit formula you found in part a to find a6.
Therefore, after 6 forwards 16,807 people will havereceived the e-mail.
19. BIOLOGY A certain bacteria divides every 15 minutes to produce two complete bacteria. a. If an initial colony contains a population of b0
bacteria, write an equation that will determine the
number of bacteria bt present after t hours.
b. Suppose a Petri dish contains 12 bacteria. Use the equation found in part a to determine the number of bacteria present 4 hours later.
SOLUTION: a. Initially, there is 1 bacterium. After 15 minutes, there will be 2 bacteria, after 30 minutes there will be 4 bacteria, after 45 minutes there will be 8 bacteria, and after 1 hour there will be 16 bacteria.
So, in terms of hours, b0 = 1 and b1 = 16. Find the
common ratio.
16 ÷ 1 = 16 Write an explicit formula using r = 16.
b. Substitute b0 = 12 and t = 4 into the equation you
found in part a.
Find the specified term for each geometric sequence or sequence with the given characteristics.
20. a9 for 60, 30, 15, …
SOLUTION: First, find the common ratio.
30 ÷ 60 =
15 ÷ 30 =
Use the formula for the nth term of a geometric
sequence to find a9.
21. a4 for 7, 14, 28, …
SOLUTION: First, find the common ratio. 14 ÷ 7 = 2 28 ÷ 14 = 2 Use the formula for the nth term of a geometric
sequence to find a4.
22. a5 for 3, 1, , …
SOLUTION: First, find the common ratio.
1 ÷ 3 =
÷ 1 =
Use the formula for the nth term of a geometric
sequence to find a5.
23. a6 for 540, 90, 15, …
SOLUTION: First, find the common ratio.
90 ÷ 540 =
15 ÷ 90 =
Use the formula for the nth term of a geometric
sequence to find a6.
24. a7 if a3 = 24 and r = 0.5
SOLUTION:
Use the values of a3 and r to find a2.
Next, use the values of a2 and r to find a1.
Use the formula for the nth term of a geometric
sequence to find a7.
Another method would be to consider that the 7th term is 4 terms from the 3rd term. Therefore,
multiply the 3rd term by r4.
25. a6 if a3 = 32 and r = –0.5
SOLUTION:
Use the values of a3 and r to find a2.
Next, use the values of a2 and r to find a1.
Use the formula for the nth term of a geometric
sequence to find a6.
Another method would be to consider that the 6th term is 3 terms from the 3rd term. Therefore,
multiply the 3rd term by r3.
26. a6 if a1 = 16,807 and r =
SOLUTION: Use the formula for the nth term of a geometric
sequence to find a6.
27. a8 if a1 = 4096 and r =
SOLUTION: Use the formula for the nth term of a geometric
sequence to find a8.
28. ACCOUNTING Julian Rockman is an accountantfor a small company. On January 1, 2009, the company purchased $50,000 worth of computers, printers, scanners, and hardware. Because this equipment is a company asset, Mr. Rockman needsto determine how much the computer equipment is presently worth. He estimates that the computer equipment depreciates at a rate of 45% per year. What value should Mr. Rockman assign the equipment in his 2014 year-end accounting report?
SOLUTION:
The equipment is originally worth $50,000, so a1 =
50,000. Because the equipment depreciates at a rate of 45% per year, the value of the equipment ona given year will be 100% – 45% or 55% of the value the previous year. So, r = 0.55. The first term
a1 corresponds to the year 2009, so the year 2014
corresponds to a6.
Use the formula for the nth term of a geometric
sequence to find the a6.
Therefore, the value of the equipment in 2014 is about $2516.42.
29. Find the sixth term of a geometric sequence with a first term of 9 and a common ratio of 2.
SOLUTION: Use the formula for the nth term of a geometric
sequence to find the a6.
30. If r = 4 and a8 = 100, what is the first term of the geometric sequence?
SOLUTION:
Substitute a8 = 100, r = 4, and n = 8 into the
formula for the nth term of a geometric sequence
to find the a1.
31. X GAMES Refer to the beginning of the lesson. The X Games netted approximately $40 million in revenue in 2002. If the X Games continue to generate 13% more revenue each year, how much revenue will the X Games generate in 2020?
SOLUTION: The X Games netted about $40,000,000 in 2002, so
a1 = $40,000,000. Because the games generate
13% or 0.13 more revenue each year, the amount of revenue generated on a given year will be 1.13 times the revenue from the previous year. So, r =
1.13. The first term a1 corresponds to the year
2002, so the year 2020 corresponds to a19.
Use the formula for the nth term of a geometric
sequence to find the a19.
Therefore, the X Games will generate about $360.97 million in 2020.
Find the indicated geometric means for each pair of nonconsecutive terms.
32. 4 and 256; 2 means
SOLUTION: The sequence will resemble 4, __?__ , __?__ , 256. Note that a1 = 4, n = 4, and a4 = 256. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is 4. Use r to find the geometric means. 4(4) = 16 16(4) = 64 Therefore, a sequence with two geometric means between 4 and 256 is 4, 16, 64, 256.
33. 256 and 81; 3 means
SOLUTION: The sequence will resemble 256, __?__ , __?__ ,
__?__ , 81. Note that a1 = 256, n = 5, and a5 = 81.
Find the common ratio using the nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
r =
r =
Therefore, a sequence with three geometric means between 256 and 81 is 256, –192, 144, –108, 81 or 256, 192, 144, 108, 81.
34. and 7; 1 mean
SOLUTION:
The sequence will resemble , __?__ , 7. Note
that a1 = , n = 3, and a3 = 7. Find the common
ratio using the nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
Therefore, a sequence with one geometric mean
between and 7 is , –2, 7 or , 2, 7.
35. –2 and 54; 2 means
SOLUTION: The sequence will resemble –2, __?__ , __?__ , 54. Note that a1 = –2, n = 4, and a4 = 54. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is –3. Use r to find the geometric means. –2(–3) = 6 6(–3) = –18 Therefore, a sequence with two geometric means between –2 and 54 is –2, 6, –18, 54.
36. 1 and 27; 2 means
SOLUTION: The sequence will resemble 1, __?__ , __?__ , 27. Note that a1 = 1, n = 4, and a4 = 27. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is 3. Use r to find the geometric means. 1(3) = 3 3(3) = 9 Therefore, a sequence with two geometric means between 1 and 27 is 1, 3, 9, 27.
37. 48 and −750; 2 means
SOLUTION: The sequence will resemble 48, __?__ , __?__ , –750. Note that a1 = 48, n = 4, and a4 = 48. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
= –120
= 300
Therefore, a sequence with two geometric means between 48 and –750 is 48, –120, 300, –750.
38. i and −1; 4 means
SOLUTION: The sequence will resemble i, __?__ , __?__ , __?__ , __?__ , –1. Note that a1 = i, n = 6, and a6 = –1. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is i. Use r to find the geometric means. i(i) = –1 –1(i) = –i –i(i) = 1 1(i) = i Therefore, a sequence with two geometric means
between i and −1 is i, –1, –i, 1, i, –1.
39. t 8 and t – 7; 4 means
SOLUTION:
The sequence will resemble t8, __?__ , __?__ ,
__?__ , __?__ , t – 7
.
Note that a1 = t8, n = 6, and a6 = t
– 7. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
= t5
= t2
= t–1
= t–4
Therefore, a sequence with two geometric means
between t8 and t
– 7 is t
8, t
5, t
2, t–1
, t–4
, t – 7
.
Find the sum of each geometric series described.
40. first six terms of 3 + 9 + 27 + …
SOLUTION: First, find the common ratio. 9 ÷ 3 = 3 27 ÷ 9 = 3 The common ratio is 3. Use Formula 1 for the sum of a finite geometric series.
41. first nine terms of 0.5 + (–1) + 2 + …
SOLUTION: First, find the common ratio. –1 ÷ 0.5 or –2 2 ÷ –1 or –2 The common ratio is –2. Use Formula 1 for the sum of a finite geometric series.
42. first eight terms of 2 + 2 + 6 + …
SOLUTION: First, find the common ratio.
2 ÷ 2 or
6 ÷ 2 or
The common ratio is . Use Formula 1 for the sum of a finite geometric series.
43. first n terms of a1 = 4, an = 2000, r = −3
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
44. [Contains update not in print edition] first n terms of a1 = 5, an = 1,310,720, r = 4
SOLUTION: [Solution for updated problem] Use Formula 2 for the nth partial sum of a geometric series.
45. first n terms of a1 = 3, an = 46,875, r = −5
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
46. first n terms of a1 = −8, an = −256, r = 2
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
47. first n terms of a1 = −36, an = 972, r = 7
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
Find each sum.
48.
SOLUTION:
Find n, a1, and r.
Substitute n = 6, a1 = 5, and r = 2 into the formula
for the sum of a finite geometric series.
49.
SOLUTION:
Find n, a1, and r.
Substitute n = 5, a1 = −4, and r = 3 into the formula
for the sum of a finite geometric series.
50.
SOLUTION:
Find n, a1, and r.
Substitute n = 5, a1 = 1, and r = −3 into the formula
for the sum of a finite geometric series.
51.
SOLUTION:
Find n, a1, and r.
Substitute n = 6, a1 = 2, and r = 1.4 into the
formula for the sum of a finite geometric series.
52.
SOLUTION:
Find n, a1, and r.
Substitute n = 6, a1 = 100, and r = into the
formula for the sum of a finite geometric series.
53.
SOLUTION:
Find n, a1, and r.
Substitute n = 9, a1 = , and r = −3 into the
formula for the sum of a finite geometric series.
54.
SOLUTION:
Find n, a1, and r.
Substitute n = 7, a1 = 144, and r = into the
formula for the sum of a finite geometric series.
55.
SOLUTION:
Find n, a1, and r.
Substitute n = 20, a1 = 3, and r = 2 into the formula
for the sum of a finite geometric series.
If possible, find the sum of each infinite geometric series.
56. + + + …
SOLUTION: First, find the common ratio.
÷ =
÷ =
The common ratio r is 1. Therefore, this infinite geometric series has no sum.
58. 18 + (–27) + 40.5 + ...
SOLUTION: First, find the common ratio. –27 ÷ 18 = –1.5 40.5 ÷ –27 = –1.5 The common ratio r is > 1. Therefore, this infinite geometric series has no sum.
59. 12 + (–7.2) + 4.32 + ...
SOLUTION: First, find the common ratio. –7.2 ÷ 12 = –0.6 4.32 ÷ –7.2 = –0.6 The common ratio r is 1. Therefore, this infinite geometric series has no sum.
59. 12 + (–7.2) + 4.32 + ...
SOLUTION: First, find the common ratio. –7.2 ÷ 12 = –0.6 4.32 ÷ –7.2 = –0.6 The common ratio r is
-
Determine the common ratio, and find the next three terms of each geometric sequence.
1.
SOLUTION: First, find the common ratio.
= –2
= –2
The common ratio is –2. Multiply the third term by –2 to find the fourth term, and so on. –1(–2) = 2 2(–2) = –4 –4(–2) = 8 Therefore, the next three terms are 2, –4, and 8.
2.
SOLUTION: First, find the common ratio.
The common ratio is . Multiply the third term
by to find the fourth term, and so on.
Therefore, the next three terms are – , ,
and – .
3. 0.5, 0.75, 1.125, …
SOLUTION: First, find the common ratio. 0.75 ÷ 0.5 = 1.5 1.125 ÷ 0.75 = 1.5 The common ratio is 1.5. Multiply the third term by 1.5 to find the fourth term, and so on. 1.125(1.5) = 1.6875 1.6875(1.5) = 2.53125 2.53125(1.5) = 3.796875 Therefore, the next three terms are 1.6875, 2.53125, and 3.796875.
4. 8, 20, 50, …
SOLUTION: First, find the common ratio. 20 ÷ 8 or 2.5 50 ÷ 20 or 2.5 The common ratio is 2.5. Multiply the third term by 2.5 to find the fourth term, and so on. 50(2.5) = 125 125(2.5) = 312.5 312.5(2.5) = 781.25 Therefore, the next three terms are 125, 312.5, and 781.25.
5. 2x, 10x, 50x, …
SOLUTION: First, find the common ratio. 10x ÷ 2x = 5 50x ÷ 10x = 5 The common ratio is 5. Multiply the third term by 5 to find the fourth term, and so on. 5(50x) = 250x 5(250x) =1250x 5(1250x) =6250x Therefore, the next three terms are 250x, 1250x, and 6250x.
6. 64x, 16x, 4x, …
SOLUTION: First, find the common ratio.
16x ÷ 64x =
4x ÷ 16x =
The common ratio is . Multiply the third term by
to find the fourth term, and so on.
Therefore, the next three terms are x, x, and
x.
7. x + 5, 3x + 15, 9x + 45, …
SOLUTION: First, find the common ratio.
The common ratio is 3. Multiply the third term by 3 to find the fourth term, and so on. 3(9x + 45) = 27x +135 3(27x +135) = 81x + 405 3(81x + 405) = 243x + 1215 Therefore, the next three terms are 27x +135, 81x + 405, and 243x + 1215.
8. –9 – y , 27 + 3y , –81 – 9y , …
SOLUTION: First, find the common ratio.
The common ratio is –3. Multiply the third term by –3 to find the fourth term, and so on. –3(–81 – 9y) = 243 + 27y –3(243 + 27y) = –729 – 81y –3(–729 – 81y) = 2187 + 243y Therefore, the next three terms are 243 + 27y , –729 – 81y , and 2187 + 243y .
9. GEOMETRY Consider a sequence of circles withdiameters that form a geometric sequence: d1, d2,
d3, d4, d5.
a. Show that the sequence of circumferences of the circles is also geometric. Identify r. b. Show that the sequence of areas of the circles isalso geometric. Identify the common ratio.
SOLUTION: a. Sample answer: The circumference of a circle isgiven by C = πd. So, the sequence of circumferences of the circles is πd1, πd2, πd3, πd4,
πd5.
Find the common ratio.
b. Sample answer: The area of a circle is given by
C = πr2 or So, the sequence of areas
of the circles is
Find the common ratio.
Write an explicit formula and a recursive formula for finding the nth term of each geometric sequence.
10. 36, 12, 4, …
SOLUTION: First, find the common ratio.
12 ÷ 36 =
4 ÷ 12 =
For an explicit formula, substitute a1 = 36 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 36, an =
11. 64, 16, 4, …
SOLUTION: First, find the common ratio.
16 ÷ 64 =
4 ÷ 16 =
For an explicit formula, substitute a1 = 64 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 64, an =
12. −2, 10, −50, …
SOLUTION: First, find the common ratio. 10 ÷ –2 = –5 –50 ÷ 10 = –5 For an explicit formula, substitute a1 = –2 and r = –
5 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = –2, an =
13. 4, −12, 36, …
SOLUTION: First, find the common ratio. –12 ÷ 4 = –3 36 ÷ –12 = –3 For an explicit formula, substitute a1 = 4 and r = –3
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 4, an =
14. 4, 8, 16, …
SOLUTION: First, find the common ratio. 8 ÷ 4 = 2 16 ÷ 8 = 2 For an explicit formula, substitute a1 = 4 and r = 2
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 4, an =
15. 20, 30, 45, …
SOLUTION: First, find the common ratio. 30 ÷ 20 = 1.5 45 ÷ 30 = 1.5 For an explicit formula, substitute a1 = 20 and r =
1.5 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 20, an =
16. 15, 5, , …
SOLUTION: First, find the common ratio.
5 ÷ 15 =
÷ 5 =
For an explicit formula, substitute a1 = 15 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 15, an =
17. , , , …
SOLUTION: First, find the common ratio.
÷ = 2
÷ = 2
For an explicit formula, substitute a1 = and r =
2 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = , an =
18. CHAIN E-MAIL Melina receives a chain e-mail that she forwards to 7 of her friends. Each of her friends forwards it to 7 of their friends.
a. Write an explicit formula for the pattern. b. How many will receive the e-mail after 6 forwards?
SOLUTION: Melina receives a chain e-mail, forwards it to 7 friends, and each friend forwards it to 7 friends.
Therefore, a1 = 1, a2 = 7, and a3 = 49. The
common ratio is 7. For an explicit formula, substitute a1 = 1 and r = 7
in the nth term formula.
b. Use the explicit formula you found in part a to find a6.
Therefore, after 6 forwards 16,807 people will havereceived the e-mail.
19. BIOLOGY A certain bacteria divides every 15 minutes to produce two complete bacteria. a. If an initial colony contains a population of b0
bacteria, write an equation that will determine the
number of bacteria bt present after t hours.
b. Suppose a Petri dish contains 12 bacteria. Use the equation found in part a to determine the number of bacteria present 4 hours later.
SOLUTION: a. Initially, there is 1 bacterium. After 15 minutes, there will be 2 bacteria, after 30 minutes there will be 4 bacteria, after 45 minutes there will be 8 bacteria, and after 1 hour there will be 16 bacteria.
So, in terms of hours, b0 = 1 and b1 = 16. Find the
common ratio.
16 ÷ 1 = 16 Write an explicit formula using r = 16.
b. Substitute b0 = 12 and t = 4 into the equation you
found in part a.
Find the specified term for each geometric sequence or sequence with the given characteristics.
20. a9 for 60, 30, 15, …
SOLUTION: First, find the common ratio.
30 ÷ 60 =
15 ÷ 30 =
Use the formula for the nth term of a geometric
sequence to find a9.
21. a4 for 7, 14, 28, …
SOLUTION: First, find the common ratio. 14 ÷ 7 = 2 28 ÷ 14 = 2 Use the formula for the nth term of a geometric
sequence to find a4.
22. a5 for 3, 1, , …
SOLUTION: First, find the common ratio.
1 ÷ 3 =
÷ 1 =
Use the formula for the nth term of a geometric
sequence to find a5.
23. a6 for 540, 90, 15, …
SOLUTION: First, find the common ratio.
90 ÷ 540 =
15 ÷ 90 =
Use the formula for the nth term of a geometric
sequence to find a6.
24. a7 if a3 = 24 and r = 0.5
SOLUTION:
Use the values of a3 and r to find a2.
Next, use the values of a2 and r to find a1.
Use the formula for the nth term of a geometric
sequence to find a7.
Another method would be to consider that the 7th term is 4 terms from the 3rd term. Therefore,
multiply the 3rd term by r4.
25. a6 if a3 = 32 and r = –0.5
SOLUTION:
Use the values of a3 and r to find a2.
Next, use the values of a2 and r to find a1.
Use the formula for the nth term of a geometric
sequence to find a6.
Another method would be to consider that the 6th term is 3 terms from the 3rd term. Therefore,
multiply the 3rd term by r3.
26. a6 if a1 = 16,807 and r =
SOLUTION: Use the formula for the nth term of a geometric
sequence to find a6.
27. a8 if a1 = 4096 and r =
SOLUTION: Use the formula for the nth term of a geometric
sequence to find a8.
28. ACCOUNTING Julian Rockman is an accountantfor a small company. On January 1, 2009, the company purchased $50,000 worth of computers, printers, scanners, and hardware. Because this equipment is a company asset, Mr. Rockman needsto determine how much the computer equipment is presently worth. He estimates that the computer equipment depreciates at a rate of 45% per year. What value should Mr. Rockman assign the equipment in his 2014 year-end accounting report?
SOLUTION:
The equipment is originally worth $50,000, so a1 =
50,000. Because the equipment depreciates at a rate of 45% per year, the value of the equipment ona given year will be 100% – 45% or 55% of the value the previous year. So, r = 0.55. The first term
a1 corresponds to the year 2009, so the year 2014
corresponds to a6.
Use the formula for the nth term of a geometric
sequence to find the a6.
Therefore, the value of the equipment in 2014 is about $2516.42.
29. Find the sixth term of a geometric sequence with a first term of 9 and a common ratio of 2.
SOLUTION: Use the formula for the nth term of a geometric
sequence to find the a6.
30. If r = 4 and a8 = 100, what is the first term of the geometric sequence?
SOLUTION:
Substitute a8 = 100, r = 4, and n = 8 into the
formula for the nth term of a geometric sequence
to find the a1.
31. X GAMES Refer to the beginning of the lesson. The X Games netted approximately $40 million in revenue in 2002. If the X Games continue to generate 13% more revenue each year, how much revenue will the X Games generate in 2020?
SOLUTION: The X Games netted about $40,000,000 in 2002, so
a1 = $40,000,000. Because the games generate
13% or 0.13 more revenue each year, the amount of revenue generated on a given year will be 1.13 times the revenue from the previous year. So, r =
1.13. The first term a1 corresponds to the year
2002, so the year 2020 corresponds to a19.
Use the formula for the nth term of a geometric
sequence to find the a19.
Therefore, the X Games will generate about $360.97 million in 2020.
Find the indicated geometric means for each pair of nonconsecutive terms.
32. 4 and 256; 2 means
SOLUTION: The sequence will resemble 4, __?__ , __?__ , 256. Note that a1 = 4, n = 4, and a4 = 256. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is 4. Use r to find the geometric means. 4(4) = 16 16(4) = 64 Therefore, a sequence with two geometric means between 4 and 256 is 4, 16, 64, 256.
33. 256 and 81; 3 means
SOLUTION: The sequence will resemble 256, __?__ , __?__ ,
__?__ , 81. Note that a1 = 256, n = 5, and a5 = 81.
Find the common ratio using the nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
r =
r =
Therefore, a sequence with three geometric means between 256 and 81 is 256, –192, 144, –108, 81 or 256, 192, 144, 108, 81.
34. and 7; 1 mean
SOLUTION:
The sequence will resemble , __?__ , 7. Note
that a1 = , n = 3, and a3 = 7. Find the common
ratio using the nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
Therefore, a sequence with one geometric mean
between and 7 is , –2, 7 or , 2, 7.
35. –2 and 54; 2 means
SOLUTION: The sequence will resemble –2, __?__ , __?__ , 54. Note that a1 = –2, n = 4, and a4 = 54. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is –3. Use r to find the geometric means. –2(–3) = 6 6(–3) = –18 Therefore, a sequence with two geometric means between –2 and 54 is –2, 6, –18, 54.
36. 1 and 27; 2 means
SOLUTION: The sequence will resemble 1, __?__ , __?__ , 27. Note that a1 = 1, n = 4, and a4 = 27. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is 3. Use r to find the geometric means. 1(3) = 3 3(3) = 9 Therefore, a sequence with two geometric means between 1 and 27 is 1, 3, 9, 27.
37. 48 and −750; 2 means
SOLUTION: The sequence will resemble 48, __?__ , __?__ , –750. Note that a1 = 48, n = 4, and a4 = 48. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
= –120
= 300
Therefore, a sequence with two geometric means between 48 and –750 is 48, –120, 300, –750.
38. i and −1; 4 means
SOLUTION: The sequence will resemble i, __?__ , __?__ , __?__ , __?__ , –1. Note that a1 = i, n = 6, and a6 = –1. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is i. Use r to find the geometric means. i(i) = –1 –1(i) = –i –i(i) = 1 1(i) = i Therefore, a sequence with two geometric means
between i and −1 is i, –1, –i, 1, i, –1.
39. t 8 and t – 7; 4 means
SOLUTION:
The sequence will resemble t8, __?__ , __?__ ,
__?__ , __?__ , t – 7
.
Note that a1 = t8, n = 6, and a6 = t
– 7. Find the
common ratio using nth term for a geometric sequence formula.
The common ratio is . Use r to find the
geometric means.
= t5
= t2
= t–1
= t–4
Therefore, a sequence with two geometric means
between t8 and t
– 7 is t
8, t
5, t
2, t–1
, t–4
, t – 7
.
Find the sum of each geometric series described.
40. first six terms of 3 + 9 + 27 + …
SOLUTION: First, find the common ratio. 9 ÷ 3 = 3 27 ÷ 9 = 3 The common ratio is 3. Use Formula 1 for the sum of a finite geometric series.
41. first nine terms of 0.5 + (–1) + 2 + …
SOLUTION: First, find the common ratio. –1 ÷ 0.5 or –2 2 ÷ –1 or –2 The common ratio is –2. Use Formula 1 for the sum of a finite geometric series.
42. first eight terms of 2 + 2 + 6 + …
SOLUTION: First, find the common ratio.
2 ÷ 2 or
6 ÷ 2 or
The common ratio is . Use Formula 1 for the sum of a finite geometric series.
43. first n terms of a1 = 4, an = 2000, r = −3
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
44. [Contains update not in print edition] first n terms of a1 = 5, an = 1,310,720, r = 4
SOLUTION: [Solution for updated problem] Use Formula 2 for the nth partial sum of a geometric series.
45. first n terms of a1 = 3, an = 46,875, r = −5
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
46. first n terms of a1 = −8, an = −256, r = 2
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
47. first n terms of a1 = −36, an = 972, r = 7
SOLUTION: Use Formula 2 for the nth partial sum of a geometric series.
Find each sum.
48.
SOLUTION:
Find n, a1, and r.
Substitute n = 6, a1 = 5, and r = 2 into the formula
for the sum of a finite geometric series.
49.
SOLUTION:
Find n, a1, and r.
Substitute n = 5, a1 = −4, and r = 3 into the formula
for the sum of a finite geometric series.
50.
SOLUTION:
Find n, a1, and r.
Substitute n = 5, a1 = 1, and r = −3 into the formula
for the sum of a finite geometric series.
51.
SOLUTION:
Find n, a1, and r.
Substitute n = 6, a1 = 2, and r = 1.4 into the
formula for the sum of a finite geometric series.
52.
SOLUTION:
Find n, a1, and r.
Substitute n = 6, a1 = 100, and r = into the
formula for the sum of a finite geometric series.
53.
SOLUTION:
Find n, a1, and r.
Substitute n = 9, a1 = , and r = −3 into the
formula for the sum of a finite geometric series.
54.
SOLUTION:
Find n, a1, and r.
Substitute n = 7, a1 = 144, and r = into the
formula for the sum of a finite geometric series.
55.
SOLUTION:
Find n, a1, and r.
Substitute n = 20, a1 = 3, and r = 2 into the formula
for the sum of a finite geometric series.
If possible, find the sum of each infinite geometric series.
56. + + + …
SOLUTION: First, find the common ratio.
÷ =
÷ =
The common ratio r is 1. Therefore, this infinite geometric series has no sum.
58. 18 + (–27) + 40.5 + ...
SOLUTION: First, find the common ratio. –27 ÷ 18 = –1.5 40.5 ÷ –27 = –1.5 The common ratio r is > 1. Therefore, this infinite geometric series has no sum.
59. 12 + (–7.2) + 4.32 + ...
SOLUTION: First, find the common ratio. –7.2 ÷ 12 = –0.6 4.32 ÷ –7.2 = –0.6 The common ratio r is 1. Therefore, this infinite geometric series has no sum.
59. 12 + (–7.2) + 4.32 + ...
SOLUTION: First, find the common ratio. –7.2 ÷ 12 = –0.6 4.32 ÷ –7.2 = –0.6 The common ratio r is
-
Determine the common ratio, and find the next three terms of each geometric sequence.
1.
SOLUTION: First, find the common ratio.
= –2
= –2
The common ratio is –2. Multiply the third term by –2 to find the fourth term, and so on. –1(–2) = 2 2(–2) = –4 –4(–2) = 8 Therefore, the next three terms are 2, –4, and 8.
2.
SOLUTION: First, find the common ratio.
The common ratio is . Multiply the third term
by to find the fourth term, and so on.
Therefore, the next three terms are – , ,
and – .
3. 0.5, 0.75, 1.125, …
SOLUTION: First, find the common ratio. 0.75 ÷ 0.5 = 1.5 1.125 ÷ 0.75 = 1.5 The common ratio is 1.5. Multiply the third term by 1.5 to find the fourth term, and so on. 1.125(1.5) = 1.6875 1.6875(1.5) = 2.53125 2.53125(1.5) = 3.796875 Therefore, the next three terms are 1.6875, 2.53125, and 3.796875.
4. 8, 20, 50, …
SOLUTION: First, find the common ratio. 20 ÷ 8 or 2.5 50 ÷ 20 or 2.5 The common ratio is 2.5. Multiply the third term by 2.5 to find the fourth term, and so on. 50(2.5) = 125 125(2.5) = 312.5 312.5(2.5) = 781.25 Therefore, the next three terms are 125, 312.5, and 781.25.
5. 2x, 10x, 50x, …
SOLUTION: First, find the common ratio. 10x ÷ 2x = 5 50x ÷ 10x = 5 The common ratio is 5. Multiply the third term by 5 to find the fourth term, and so on. 5(50x) = 250x 5(250x) =1250x 5(1250x) =6250x Therefore, the next three terms are 250x, 1250x, and 6250x.
6. 64x, 16x, 4x, …
SOLUTION: First, find the common ratio.
16x ÷ 64x =
4x ÷ 16x =
The common ratio is . Multiply the third term by
to find the fourth term, and so on.
Therefore, the next three terms are x, x, and
x.
7. x + 5, 3x + 15, 9x + 45, …
SOLUTION: First, find the common ratio.
The common ratio is 3. Multiply the third term by 3 to find the fourth term, and so on. 3(9x + 45) = 27x +135 3(27x +135) = 81x + 405 3(81x + 405) = 243x + 1215 Therefore, the next three terms are 27x +135, 81x + 405, and 243x + 1215.
8. –9 – y , 27 + 3y , –81 – 9y , …
SOLUTION: First, find the common ratio.
The common ratio is –3. Multiply the third term by –3 to find the fourth term, and so on. –3(–81 – 9y) = 243 + 27y –3(243 + 27y) = –729 – 81y –3(–729 – 81y) = 2187 + 243y Therefore, the next three terms are 243 + 27y , –729 – 81y , and 2187 + 243y .
9. GEOMETRY Consider a sequence of circles withdiameters that form a geometric sequence: d1, d2,
d3, d4, d5.
a. Show that the sequence of circumferences of the circles is also geometric. Identify r. b. Show that the sequence of areas of the circles isalso geometric. Identify the common ratio.
SOLUTION: a. Sample answer: The circumference of a circle isgiven by C = πd. So, the sequence of circumferences of the circles is πd1, πd2, πd3, πd4,
πd5.
Find the common ratio.
b. Sample answer: The area of a circle is given by
C = πr2 or So, the sequence of areas
of the circles is
Find the common ratio.
Write an explicit formula and a recursive formula for finding the nth term of each geometric sequence.
10. 36, 12, 4, …
SOLUTION: First, find the common ratio.
12 ÷ 36 =
4 ÷ 12 =
For an explicit formula, substitute a1 = 36 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 36, an =
11. 64, 16, 4, …
SOLUTION: First, find the common ratio.
16 ÷ 64 =
4 ÷ 16 =
For an explicit formula, substitute a1 = 64 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 64, an =
12. −2, 10, −50, …
SOLUTION: First, find the common ratio. 10 ÷ –2 = –5 –50 ÷ 10 = –5 For an explicit formula, substitute a1 = –2 and r = –
5 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = –2, an =
13. 4, −12, 36, …
SOLUTION: First, find the common ratio. –12 ÷ 4 = –3 36 ÷ –12 = –3 For an explicit formula, substitute a1 = 4 and r = –3
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 4, an =
14. 4, 8, 16, …
SOLUTION: First, find the common ratio. 8 ÷ 4 = 2 16 ÷ 8 = 2 For an explicit formula, substitute a1 = 4 and r = 2
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 4, an =
15. 20, 30, 45, …
SOLUTION: First, find the common ratio. 30 ÷ 20 = 1.5 45 ÷ 30 = 1.5 For an explicit formula, substitute a1 = 20 and r =
1.5 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 20, an =
16. 15, 5, , …
SOLUTION: First, find the common ratio.
5 ÷ 15 =
÷ 5 =
For an explicit formula, substitute a1 = 15 and r =
in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = 15, an =
17. , , , …
SOLUTION: First, find the common ratio.
÷ = 2
÷ = 2
For an explicit formula, substitute a1 = and r =
2 in the nth term formula.
For a recursive formula, state the first term a1.
Then indicate that the next term is the product of
the first term an – 1 and r.
a1 = , an =
18. CHAIN E-MAIL Melina receives a chain e-mail that she forwards to 7 of her friends. Each of her friends forwards it to 7 of their friends.
a. Write an explicit formula for the pattern. b. How many will receive the e-mail after 6 forwards?
SOLUTION: Melina receives a chain e-mail, forwards it to 7 friends, and each friend forwards it to 7 friends.
Therefore, a1 = 1, a2 = 7, and a3 = 49. The
common ratio is 7. For an explicit formula, substitute a1 = 1 and r = 7
in the nth term formula.
b. Use the explicit formula you found in part a to find a6.
Therefore, after 6 forwards 16,807 people will havereceived the e-mail.
19. BIOLOGY A certain bacteria divides every 15 minutes to produce two complete bacteria. a. If an initial colony contains a population of b0
bacteria, write an equation that will determine the
number of bacteria bt present after t hours.
b. Suppose a Petri dish contains 12 bacteria. Use the equation found in part a to determine the number of bacteria present 4 hours later.
SOLUTION: a. Initially, there is 1 bacterium. After 15 minutes, there will be 2 bacteria, after 30 minutes there will be 4 bacteria, after 45 minutes there will be 8 bacteria, and after 1 hour there will be 16 bacteria.
So, in terms of hours, b0 = 1 and b1 = 16. Find the
common ratio.
16 ÷ 1 = 16 Write an explicit formula using r = 16.
b. Substitute b0 = 12 and t = 4 into the equation you
found in part a.
Find the specified term for each geometric sequence or sequence with the given characteristics.
20. a9 for 60, 30, 15, …
SOLUTION: First, find the common ratio.
30 ÷ 60 =
15 ÷ 30 =
Use the formula for the nth term of a geometric
sequence to find a9.
21. a4 for 7, 14, 28, …
SOLUTION: First, find the common ratio. 14 ÷ 7 = 2 28 ÷ 14 = 2 Use the formula for the nth term of a geometric
sequence to find a4.
22. a5 for 3, 1, , …
SOLUTION: First, find the common ratio.
1 ÷ 3 =
÷ 1 =
Use the formula for the nth term of a geometric
sequence to find a5.
23. a6 for 540, 90, 15, …
SOLUTION: First, find the common ratio.
90 ÷ 540 =
15 ÷ 90 =
Use the formula for the nth term of a geometric
sequence to find a6.
24. a7 if a3 = 24 and r = 0.5
SOLUTION:
Use the values of a3 and r to find a2.
Next, use the values of a2 and r to find a1.
Use the formula for the nth term of a geometric
sequence to find a7.
Another method would be to consider that the 7th term is 4 terms from the 3rd term. Therefore,
multiply the 3rd term by r4.
25. a6 if a3 = 32 and r = –0.5
SOLUTION:
Use the values of a3 and r to find a2.
Next, use the values of a2 and r to find a1.
Use the formula for the nth term of a geometric
sequence to find a6.
Another method would be to consider that the 6th term is 3 terms from the 3rd term. Therefore,
multiply the 3rd term by r3.
26. a6 if a1 = 16,807 and r =
SOLUTION: Use the formula for the nth term of a geometric
sequence to find a6.
27. a8 if a1 = 4096 and r =
SOLUTION: Use the formula for the nth term of a geometric
sequence to find a8.
28. ACCOUNTING Julian Rockman is an accountantfor a small company. On January 1, 2009, the company purchased $50,000 worth of computers, printers, scanners, and hardware. Because this equipment