solutions to exercises, section 3park/fall2013/precalculus/3.2sol.pdf · instructor’s solutions...
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Instructor’s Solutions Manual, Section 3.2 Exercise 1
Solutions to Exercises, Section 3.2
For Exercises 1–16, evaluate the indicated expression. Do not use acalculator for these exercises.
1. log2 64
solution If we let x = log2 64, then x is the number such that
64 = 2x.
Because 64 = 26, we see that x = 6. Thus log2 64 = 6.
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Instructor’s Solutions Manual, Section 3.2 Exercise 2
2. log2 1024
solution If we let x = log2 1024, then x is the number such that
1024 = 2x.
Because 1024 = 210, we see that x = 10. Thus log2 1024 = 10.
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Instructor’s Solutions Manual, Section 3.2 Exercise 3
3. log21
128
solution If we let x = log21
128 , then x is the number such that
1128 = 2x.
Because 1128 = 1
27 = 2−7, we see that x = −7. Thus log21
128 = −7.
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Instructor’s Solutions Manual, Section 3.2 Exercise 4
4. log21
256
solution If we let x = log21
256 , then x is the number such that
1256 = 2x.
Because 1256 = 1
28 = 2−8, we see that x = −8. Thus log21
256 = −8.
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Instructor’s Solutions Manual, Section 3.2 Exercise 5
5. log4 2
solution Because 2 = 41/2, we have log4 2 = 12 .
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Instructor’s Solutions Manual, Section 3.2 Exercise 6
6. log8 2
solution Because 2 = 81/3, we have log8 2 = 13 .
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Instructor’s Solutions Manual, Section 3.2 Exercise 7
7. log4 8
solution Because 8 = 2 · 4 = 41/2 · 4 = 43/2, we have log4 8 = 32 .
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Instructor’s Solutions Manual, Section 3.2 Exercise 8
8. log8 128
solution Because 128 = 2 ·64 = 81/3 ·82 = 87/3, we have log8 128 = 73 .
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Instructor’s Solutions Manual, Section 3.2 Exercise 9
9. log 10000
solution log 10000 = log 104
= 4
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Instructor’s Solutions Manual, Section 3.2 Exercise 10
10. log 11000
solution log1
1000= log 10−3
= −3
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Instructor’s Solutions Manual, Section 3.2 Exercise 11
11. log√
1000
solution log√
1000 = log 10001/2
= log (103)1/2
= log 103/2
= 32
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Instructor’s Solutions Manual, Section 3.2 Exercise 12
12. log 1√10000
solution log1√
10000= log 10000−1/2
= log (104)−1/2
= log 10−2
= −2
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Instructor’s Solutions Manual, Section 3.2 Exercise 13
13. log2 83.1
solution log2 83.1 = log2 (23)3.1
= log2 29.3
= 9.3
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Instructor’s Solutions Manual, Section 3.2 Exercise 14
14. log8 26.3
solution log8 26.3 = log8 (81/3)6.3
= log8 82.1
= 2.1
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Instructor’s Solutions Manual, Section 3.2 Exercise 15
15. log16 32
solution log16 32 = log16 25
= log16 (24)5/4
= log16 165/4
= 54
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Instructor’s Solutions Manual, Section 3.2 Exercise 16
16. log27 81
solution log27 81 = log27 34
= log27 (33)4/3
= log27 274/3
= 43
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Instructor’s Solutions Manual, Section 3.2 Exercise 17
17. Find a number y such that log2y = 7.
solution The equation log2y = 7 implies that
y = 27 = 128.
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Instructor’s Solutions Manual, Section 3.2 Exercise 18
18. Find a number t such that log2 t = 8.
solution The equation log2 t = 8 implies that
t = 28 = 256.
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Instructor’s Solutions Manual, Section 3.2 Exercise 19
19. Find a number y such that log2y = −5.
solution The equation log2y = −5 implies that
y = 2−5 = 132 .
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Instructor’s Solutions Manual, Section 3.2 Exercise 20
20. Find a number t such that log2 t = −9.
solution The equation log2 t = −9 implies that
t = 2−9 = 1512 .
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Instructor’s Solutions Manual, Section 3.2 Exercise 21
For Exercises 21–28, find a number b such that the indicated equalityholds.
21. logb 64 = 1
solution The equation logb 64 = 1 implies that
b1 = 64.
Thus b = 64.
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Instructor’s Solutions Manual, Section 3.2 Exercise 22
22. logb 64 = 2
solution The equation logb 64 = 2 implies that
b2 = 64.
Thus b = √64 = 8.
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Instructor’s Solutions Manual, Section 3.2 Exercise 23
23. logb 64 = 3
solution The equation logb 64 = 3 implies that
b3 = 64.
Because 43 = 64, this implies that b = 4.
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Instructor’s Solutions Manual, Section 3.2 Exercise 24
24. logb 64 = 6
solution The equation logb 64 = 6 implies that
b6 = 64.
Because 26 = 64, this implies that b = 2.
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Instructor’s Solutions Manual, Section 3.2 Exercise 25
25. logb 64 = 12
solution The equation logb 64 = 12 implies that
b12 = 64.
Thus
b = 641/12
= (26)1/12
= 26/12
= 21/2
= √2.
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Instructor’s Solutions Manual, Section 3.2 Exercise 26
26. logb 64 = 18
solution The equation logb 64 = 18 implies that
b18 = 64.
Thus
b = 641/18
= (26)1/18
= 26/18
= 21/3.
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Instructor’s Solutions Manual, Section 3.2 Exercise 27
27. logb 64 = 32
solution The equation logb 64 = 32 implies that
b3/2 = 64.
Raising both sides of this equation to the 2/3 power, we get
b = 642/3
= (26)2/3
= 24
= 16.
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Instructor’s Solutions Manual, Section 3.2 Exercise 28
28. logb 64 = 65
solution The equation logb 64 = 65 implies that
b6/5 = 64.
Raising both sides of this equation to the 5/6 power, we get
b = 645/6
= (26)5/6
= 25
= 32.
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Instructor’s Solutions Manual, Section 3.2 Exercise 29
29. Find a number x such that log3(5x + 1) = 2.
solution The equation log3(5x + 1) = 2 implies that 5x + 1 = 32 = 9.Thus 5x = 8, which implies that x = 8
5 .
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Instructor’s Solutions Manual, Section 3.2 Exercise 30
30. Find a number x such that log4(3x + 1) = −2.
solution The equation log4(3x + 1) = −2 implies that3x + 1 = 4−2 = 1
16 . Thus 3x = −1516 , which implies that x = − 5
16 .
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Instructor’s Solutions Manual, Section 3.2 Exercise 31
31. Find a number x such that 13 = 102x .
solution The equation 13 = 102x implies that 2x = log 13. Thusx = log 13
2 , which is approximately equal to 0.557.
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Instructor’s Solutions Manual, Section 3.2 Exercise 32
32. Find a number x such that 59 = 103x .
solution The equation 59 = 103x implies that 3x = log 59. Thusx = log 59
3 , which is approximately equal to 0.590284.
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Instructor’s Solutions Manual, Section 3.2 Exercise 33
33. Find a number t such that
10t + 110t + 2
= 0.8.
solution Multiplying both sides of the equation above by 10t + 2, weget
10t + 1 = 0.8 · 10t + 1.6.
Solving this equation for 10t gives 10t = 3, which means thatt = log 3 ≈ 0.477121.
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Instructor’s Solutions Manual, Section 3.2 Exercise 34
34. Find a number t such that
10t + 3.810t + 3
= 1.1.
solution Multiplying both sides of the equation above by 10t + 3, weget
10t + 3.8 = 1.1 · 10t + 3.3.
Solving this equation for 10t gives 10t = 5, which means thatt = log 5 ≈ 0.69897.
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Instructor’s Solutions Manual, Section 3.2 Exercise 35
35. Find a number x such that
102x + 10x = 12.
solution Note that 102x = (10x)2. This suggests that we let y = 10x .Then the equation above can be rewritten as
y2 +y − 12 = 0.
The solutions to this equation (which can be found either by using thequadratic formula or by factoring) are y = −4 and y = 3. Thus10x = −4 or 10x = 3. However, there is no real number x such that10x = −4 (because 10x is positive for every real number x), and thuswe must have 10x = 3. Thus x = log 3 ≈ 0.477121.
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Instructor’s Solutions Manual, Section 3.2 Exercise 36
36. Find a number x such that
102x − 3 · 10x = 18.
solution Note that 102x = (10x)2. This suggests that we let y = 10x .Then the equation above can be rewritten as
y2 − 3y − 18 = 0.
The solutions to this equation (which can be found either by using thequadratic formula or by factoring) are y = −3 and y = 6. Thus10x = −3 or 10x = 6. However, there is no real number x such that10x = −3 (because 10x is positive for every real number x), and thuswe must have 10x = 6. Thus x = log 6 ≈ 0.778151.
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Instructor’s Solutions Manual, Section 3.2 Exercise 37
For Exercises 37–54, find a formula for the inverse function f−1 of theindicated function f .
37. f(x) = 3x
solution By definition of the logarithm, the inverse of f is thefunction f−1 defined by
f−1(y) = log3y.
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Instructor’s Solutions Manual, Section 3.2 Exercise 38
38. f(x) = 4.7x
solution By definition of the logarithm, the inverse of f is thefunction f−1 defined by
f−1(y) = log4.7y.
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Instructor’s Solutions Manual, Section 3.2 Exercise 39
39. f(x) = 2x−5
solution To find a formula for f−1(y), we solve the equation2x−5 = y for x. This equation means that x − 5 = log2y . Thusx = 5+ log2y . Hence
f−1(y) = 5+ log2y.
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Instructor’s Solutions Manual, Section 3.2 Exercise 40
40. f(x) = 9x+6
solution To find a formula for f−1(y), we solve the equation9x+6 = y for x. This equation means that x + 6 = log9y . Thusx = −6+ log9y . Hence
f−1(y) = −6+ log9y.
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Instructor’s Solutions Manual, Section 3.2 Exercise 41
41. f(x) = 6x + 7
solution To find a formula for f−1(y), we solve the equation6x + 7 = y for x. Subtract 7 from both sides, getting 6x = y − 7. Thisequation means that x = log6(y − 7). Hence
f−1(y) = log6(y − 7).
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Instructor’s Solutions Manual, Section 3.2 Exercise 42
42. f(x) = 5x − 3
solution To find a formula for f−1(y), we solve the equation5x − 3 = y for x. Add 3 to both sides, getting 5x = y + 3. Thisequation means that x = log5(y + 3). Hence
f−1(y) = log5(y + 3).
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Instructor’s Solutions Manual, Section 3.2 Exercise 43
43. f(x) = 4 · 5x
solution To find a formula for f−1(y), we solve the equation4 · 5x = y for x. Divide both sides by 4, getting 5x = y
4 . This equationmeans that x = log5
y4 . Hence
f−1(y) = log5y4 .
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Instructor’s Solutions Manual, Section 3.2 Exercise 44
44. f(x) = 8 · 7x
solution To find a formula for f−1(y), we solve the equation8 · 7x = y for x. Divide both sides by 8, getting 7x = y
8 . This equationmeans that x = log7
y8 . Hence
f−1(y) = log7y8 .
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Instructor’s Solutions Manual, Section 3.2 Exercise 45
45. f(x) = 2 · 9x + 1
solution To find a formula for f−1(y), we solve the equation2 · 9x + 1 = y for x. Subtract 1 from both sides, then divide both sidesby 2, getting 9x = y−1
2 . This equation means that x = log9y−1
2 . Hence
f−1(y) = log9y−1
2 .
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Instructor’s Solutions Manual, Section 3.2 Exercise 46
46. f(x) = 3 · 4x − 5
solution To find a formula for f−1(y), we solve the equation3 · 4x − 5 = y for x. Add 5 to both sides, then divide both sides by 3,getting 4x = y+5
3 . This equation means that x = log4y+5
3 . Hence
f−1(y) = log4y+5
3 .
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Instructor’s Solutions Manual, Section 3.2 Exercise 47
47. f(x) = log8 x
solution By the definition of the logarithm, the inverse of f is thefunction f−1 defined by
f−1(y) = 8y.
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Instructor’s Solutions Manual, Section 3.2 Exercise 48
48. f(x) = log3 x
solution By the definition of the logarithm, the inverse of f is thefunction f−1 defined by
f−1(y) = 3y.
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Instructor’s Solutions Manual, Section 3.2 Exercise 49
49. f(x) = log4(3x + 1)
solution To find a formula for f−1(y), we solve the equation
log4(3x + 1) = y
for x. This equation means that 3x + 1 = 4y . Solving for x, we getx = 4y−1
3 . Hence
f−1(y) = 4y − 13
.
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Instructor’s Solutions Manual, Section 3.2 Exercise 50
50. f(x) = log7(2x − 9)
solution To find a formula for f−1(y), we solve the equation
log7(2x − 9) = y
for x. This equation means that 2x − 9 = 7y . Solving for x, we getx = 7y+9
2 . Hence
f−1(y) = 7y + 92
.
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Instructor’s Solutions Manual, Section 3.2 Exercise 51
51. f(x) = 5+ 3 log6(2x + 1)
solution To find a formula for f−1(y), we solve the equation
5+ 3 log6(2x + 1) = y
for x. Subtracting 5 from both sides and then dividing by 3 gives
log6(2x + 1) = y − 53
.
This equation means that 2x + 1 = 6(y−5)/3. Solving for x, we get
x = 6(y−5)/3−12 . Hence
f−1(y) = 6(y−5)/3 − 12
.
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Instructor’s Solutions Manual, Section 3.2 Exercise 52
52. f(x) = 8+ 9 log2(4x − 7)
solution To find a formula for f−1(y), we solve the equation
8+ 9 log2(4x − 7) = y
for x. Subtracting 8 from both sides and then dividing by 9 gives
log2(4x − 7) = y − 89
.
This equation means that 4x − 7 = 2(y−8)/9. Solving for x, we get
x = 2(y−8)/9+74 . Hence
f−1(y) = 2(y−8)/9 + 74
.
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Instructor’s Solutions Manual, Section 3.2 Exercise 53
53. f(x) = logx 13
solution To find a formula for f−1(y), we solve the equationlogx 13 = y for x. This equation means that xy = 13. Raising bothsides to the power 1
y , we get x = 131/y . Hence
f−1(y) = 131/y .
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Instructor’s Solutions Manual, Section 3.2 Exercise 54
54. f(x) = log5x 6
solution To find a formula for f−1(y), we solve the equationlog5x 6 = y for x. This equation means that (5x)y = 6. Raising both
sides to the power 1y and then dividing by 5, we get x = 61/y
5 . Hence
f−1(y) = 61/y
5.
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Instructor’s Solutions Manual, Section 3.2 Exercise 55
For Exercises 55–58, find a formula for (f ◦ g)(x) assuming that f andg are the indicated functions.
55. f(x) = log6 x and g(x) = 63x
solution
(f ◦ g)(x) = f (g(x)) = f(63x) = log6 63x = 3x
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Instructor’s Solutions Manual, Section 3.2 Exercise 56
56. f(x) = log5 x and g(x) = 53+2x
solution
(f ◦ g)(x) = f (g(x)) = f(53+2x)
= log5 53+2x = 3+ 2x
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Instructor’s Solutions Manual, Section 3.2 Exercise 57
57. f(x) = 63x and g(x) = log6 x
solution
(f ◦ g)(x) = f (g(x)) = f(log6 x)
= 63 log6 x = (6log6 x)3 = x3
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Instructor’s Solutions Manual, Section 3.2 Exercise 58
58. f(x) = 53+2x and g(x) = log5 x
solution
(f ◦ g)(x) = f (g(x)) = f(log5 x)
= 53+2 log5 x = 5352 log5 x
= 125(5log5 x)2 = 125x2
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Instructor’s Solutions Manual, Section 3.2 Exercise 59
59. Find a number n such that log3(log5n) = 1.
solution The equation log3(log5n) = 1 implies that log5n = 3, whichimplies that n = 53 = 125.
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Instructor’s Solutions Manual, Section 3.2 Exercise 60
60. Find a number n such that log3(log2n) = 2.
solution The equation log3(log2n) = 2 implies that log2n = 32 = 9,which implies that n = 29 = 512.
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Instructor’s Solutions Manual, Section 3.2 Exercise 61
61. Find a number m such that log7(log8m) = 2.
solution The equation log7(log8m) = 2 implies that
log8m = 72 = 49.
The equation above now implies that
m = 849.
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Instructor’s Solutions Manual, Section 3.2 Exercise 62
62. Find a number m such that log5(log6m) = 3.
solution The equation log5(log6m) = 3 implies that
log6m = 53 = 125.
The equation above now implies that
m = 6125.
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Instructor’s Solutions Manual, Section 3.2 Exercise 63
For Exercises 63–70, evaluate the indicated quantities. Your calculator isunlikely to be able to evaluate logarithms using any of the bases in theseexercises, so you will need to use an appropriate change of base formula.
63. log2 13
solution log2 13 = log 13log 2
≈ 3.70044
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Instructor’s Solutions Manual, Section 3.2 Exercise 64
64. log4 27
solution log4 27 = log 27log 4
≈ 2.37744
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Instructor’s Solutions Manual, Section 3.2 Exercise 65
65. log13 9.72
solution log13 9.72 = log 9.72log 13
≈ 0.88664
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Instructor’s Solutions Manual, Section 3.2 Exercise 66
66. log17 12.31
solution log17 12.31 = log 12.31log 17
≈ 0.886065
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Instructor’s Solutions Manual, Section 3.2 Exercise 67
67. log9 0.23
solution log9 0.23 = log 0.23log 9
≈ −0.668878
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Instructor’s Solutions Manual, Section 3.2 Exercise 68
68. log7 0.58
solution log7 0.58 = log 0.58log 7
≈ −0.279934
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Instructor’s Solutions Manual, Section 3.2 Exercise 69
69. log4.38 7.1
solution log4.38 7.1 = log 7.1log 4.38
≈ 1.32703
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Instructor’s Solutions Manual, Section 3.2 Exercise 70
70. log5.06 99.2
solution
log5.06 99.2 = log 99.2log 5.06
≈ 2.83535
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Instructor’s Solutions Manual, Section 3.2 Problem 71
Solutions to Problems, Section 3.2
71. Explain why log3 100 is between 4 and 5.
solution Note that 34 = 81 and 35 = 243. Because 100 is between 81and 243, this implies that the number x such that 3x = 100 is between4 and 5. Thus log3 100 is between 4 and 5.
Here is another explanation: Because the function f defined byf(y) = log3y is an increasing function, we have
4 = log3 34 = log3 81 < log3 100 < log3 243 = log3 35 = 5.
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Instructor’s Solutions Manual, Section 3.2 Problem 72
72. Explain why log40 3 is between 14 and 1
3 .
solution Let x = log40 3. Thus
3 = 40x.
Raising both sides of the last equation to the power 1x gives
31/x = 40.
Note that 33 = 27 and 34 = 81. Because 40 is between 27 and 81, theequation above implies that 1
x is between 3 and 4. Thus x is between 14
and 13 .
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Instructor’s Solutions Manual, Section 3.2 Problem 73
73. Show that log2 3 is an irrational number.[Hint: Use proof by contradiction: Assume that log2 3 is equal to arational number m
n ; write out what this means, and think about evenand odd numbers.]
solution Suppose log2 3 is a rational number. Then there existpositive integers m and n such that
log2 3 = mn .
Thus2m/n = 3.
Raising both sides of the equation above to the nth power gives
2m = 3n.
However the left side of the equation above is an even number, and theright side is an odd number. This contradiction means that ourassumption that log2 3 is a rational number is false. Thus log2 3 is anirrational number.
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Instructor’s Solutions Manual, Section 3.2 Problem 74
74. Show that log 2 is irrational.
solution Suppose log 2 is a rational number. Then there existpositive integers m and n such that
log 2 = mn .
Thus2 = 10m/n.
Raising both sides of the equation above to the nth power gives
2n = 10m,
which we can rewrite as
2n = 10m = (2 · 5)m = 2m · 5m.
Thus2n−m = 5m.
However the left side of the equation above is an even number, and theright side is an odd number. This contradiction means that ourassumption that log 2 is a rational number is false. Thus log 2 is anirrational number.
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Instructor’s Solutions Manual, Section 3.2 Problem 75
75. Explain why logarithms with base 0 are not defined.
solution The equation logb y = x means that bx = y . If we allowedthe base b to equal 0, then the equation log0y = x would mean that0x = y . However, 0x = 0 for every number x > 0 and 0x is undefinedfor every number x ≤ 0. Thus the only allowable value for y would be0, and even then we would have infinitely many choices of x thatsatisfy the equation 0x = 0. The only way out of this mess is to leavelogarithms with base 0 undefined.
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Instructor’s Solutions Manual, Section 3.2 Problem 76
76. Explain why logarithms with a negative base are not defined.
solution The equation logb y = x means that bx = y . If we allowedthe base b to be negative, we could only consider values of x that arerational numbers of the form m
n , with n an odd number (because bx isnot defined for other values of x). The allowable values of y would behard to describe. Thus this definition would not be useful, and hencelogb y is left undefined when b is negative.
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Instructor’s Solutions Manual, Section 3.2 Problem 77
77. Explain why log5
√5 = 1
2 .
solution Note that51/2 = √5.
Thus log5
√5 = 1
2 .
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Instructor’s Solutions Manual, Section 3.2 Problem 78
78. Suppose a and b are positive numbers, with a �= 1 and b �= 1. Show that
loga b =1
logb a.
solution The change of base formula for logarithms states that if a,b, and y are positive numbers, then
logb y =loga yloga b
.
Taking y = a in the equation above and using the equation loga a = 1shows that
logb a =1
loga b,
which is equivalent to the equation
loga b =1
logb a.
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Instructor’s Solutions Manual, Section 3.2 Problem 79
79. Suppose b and y are positive numbers, with b �= 1 and b �= 12 . Show that
log2b y =logb y
1+ logb 2.
solution The change of base formula for logarithms states that if a,b, and y are positive numbers, then
logb y =loga yloga b
.
In this formula, replace b with 2b and replace a with b, getting
log2b y =logb y
logb(2b)= logb y
logb b + logb 2= logb y
1+ logb 2.