[email protected] mth55_lec-60_fa08_sec_9-3a_intro-to-logs.ppt 1 bruce mayer, pe chabot...
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[email protected] • MTH55_Lec-60_Fa08_sec_9-3a_Intro-to-Logs.ppt1
Bruce Mayer, PE Chabot College Mathematics
Bruce Mayer, PELicensed Electrical & Mechanical Engineer
Chabot Mathematics
§9.3a§9.3aLogarithmsLogarithms
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Bruce Mayer, PE Chabot College Mathematics
Review §Review §
Any QUESTIONS About• §9.2 → Inverse Functions
Any QUESTIONS About HomeWork• §9.2 → HW-43
9.2 MTH 55
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Bruce Mayer, PE Chabot College Mathematics
Logarithm → What is it?Logarithm → What is it?
Concept: If b > 0 and b ≠ 1, then
y = logbx is equivalent to x = by
Symbolically
x = by y = logbx
The exponent is the logarithm.
The base is the base of the logarithm.
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Bruce Mayer, PE Chabot College Mathematics
Logarithm IllustratedLogarithm Illustrated
Consider the exponential function f(x) = 3x. Like all exponential functions, f is one-to-one. Can a formula for f−1 be found? Use the 4-Step Method
f −1(x) ≡ the exponent to which we must raise 3 to get x.
y = 3x x = 3y
y ≡ the exponent to which we must raise 3 to get x.
4-Step
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Bruce Mayer, PE Chabot College Mathematics
Logarithm IllustratedLogarithm Illustrated
Now define a new symbol to replace the words “the exponent to which we must raise 3 to get x”:
log3x, read “the logarithm, base 3, of x,” or “log, base 3, of x,” means “the exponent to which we raise 3 to get x.”
Thus if f(x) = 3x, then f−1(x) = log3x. Note that f−1(9) = log39 = 2, as 2 is the exponent to which we raise 3 to get 9
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Bruce Mayer, PE Chabot College Mathematics
Example Example Evaluate Logarithms Evaluate Logarithms
Evaluate:a) log381 b) log31 c) log3(1/9)
Solution:a) Think of log381 as the exponent to which we
raise 3 to get 81. The exponent is 4. Thus, since 34 = 81, log381 = 4.
b) ask: “To what exponent do we raise 3 in order to get 1?” That exponent is 0. So, log31 = 0
c) To what exponent do we raise 3 in order to get 1/9? Since 3−2 = 1/9 we have log3(1/9) = −2
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Bruce Mayer, PE Chabot College Mathematics
The Meaning of logThe Meaning of logaaxx
For x > 0 and a a positive constant other than 1, logax is the exponent to which a must be raised in order to get x. Thus,
logax = m means am = x
or equivalently, logax is that unique exponent for which
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Bruce Mayer, PE Chabot College Mathematics
Example Example Exponential to Log Exponential to Log
Write each exponential equation in logarithmic form.
a. 43 64 b. 1
2
4
1
16c. a 2 7
Soln a. 43 64 log4 64 3
b. 1
2
4
1
16 log1 2
1
164
c. a 2 7 loga 7 2
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Bruce Mayer, PE Chabot College Mathematics
Example Example Log to Exponential Log to Exponential
Write each logarithmic equation in exponential form
a. log3 243 5 b. log2 5 x c. loga N x
Soln a. log3 243 5 243 35
b. log2 5 x 5 2x
c. loga N x N ax
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Bruce Mayer, PE Chabot College Mathematics
Example Example Evaluate Logarithms Evaluate Logarithms
Find the value of each of the following logarithmsa. log5 25 b. log2 16 c. log1 3 9
d. log7 7 e. log6 1 f. log4
1
2 Solution
a. log5 25 y 25 5y or 52 5y y 2
b. log2 16 y 16 2y or 24 2y y 4
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Bruce Mayer, PE Chabot College Mathematics
Example Example Evaluate Logarithms Evaluate Logarithms
Solution (cont.)
d. log7 7 y 7 7y or 71 7y y 1
e. log6 1 y 1 6y or 60 6y y 0
f. log4
1
2y
1
24 y or 2 1 22 y y
1
2
c. log1 3 9 y 9 1
3
y
or 32 3 y y 2
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Bruce Mayer, PE Chabot College Mathematics
Example Example Use Log Definition Use Log Definition
Solve each equation for x, y or z
a. log5 x 3 b. log3
1
27y
c. logz 1000 3 d. log2 x2 6x 10 1
a. log5 x 3
x 5 3
x 1
53 1
125
The solution set is 1
125
.
Solution
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Bruce Mayer, PE Chabot College Mathematics
Example Example Use Log Definition Use Log Definition
Solution (cont.)
b. log3
1
27y
1
273y
3 3 3y
3 y
c. logz 1000 3
1000 z3
103 z3
10 z
The solution set is 3 .
The solution set is 10 .
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Bruce Mayer, PE Chabot College Mathematics
Inverse Property of LogarithmsInverse Property of Logarithms
Recall Def: For x > 0, a > 0, and a ≠ 1,
In other words, The logarithmic function is the inverse function of the exponential function; e.g.
xaxa xxa
a loglog
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Bruce Mayer, PE Chabot College Mathematics
ShowShowLogLogaaaax
x = = xx
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Bruce Mayer, PE Chabot College Mathematics
Example Example Inverse Property Inverse Property
Evaluate:
SolutionRemember that log523 is the exponent to which 5 is raised to get 23. Raising 5 to that exponent, obtain
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Bruce Mayer, PE Chabot College Mathematics
Basic Properties of LogarithmsBasic Properties of Logarithms
For any base a > 0, with a ≠ 1, Discern from the Log Definition
1. Logaa = 1
• As 1 is the exponent to which a must be raised to obtain a (a1 = a)
2. Loga1 = 0
• As 0 is the exponent to which a must be raised to obtain 1 (a0 = 1)
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Bruce Mayer, PE Chabot College Mathematics
Graph Logarithmic FunctionGraph Logarithmic Function
Sketch the graph of y = log3x
Soln:MakeT-Table→
x y = log3x (x, y)
3–3 = 1/27 –3 (1/27, –3)
3–2 = 1/9 –2 (1/9, –2)
3–3 = 1/3 –1 (1/3, –1)
30 = 1 0 (1, 0)
31 = 3 1 (3, 1)
32 = 9 2 (9, 2)
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Bruce Mayer, PE Chabot College Mathematics
Graph Logarithmic FunctionGraph Logarithmic Function
Plot the ordered pairs and connect the dots with a smooth curve to obtain the graph of y = log3x
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Bruce Mayer, PE Chabot College Mathematics
Example Example Graph by Inverse Graph by Inverse Graph y = f(x) = 3x
Solution:Use Inverse Relationfor Logs & Exponentials
Reflect the graph of y = 3x in the line y = x to obtain the graph ofy = f−1(x) = log3x
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Bruce Mayer, PE Chabot College Mathematics
Domain of Logarithmic FcnsDomain of Logarithmic Fcns
Recall that the• Domain of f(x) = ax is (−∞, ∞)
• Range of f(x) = ax is (0, ∞)
Since the Logarithmic function is the inverse of the Exponential function,• Domain of f−1(x) = logax is (0, ∞)
• Range of f−1(x) = logax is (−∞, ∞)
Thus, the logarithms of 0 and negative numbers are NOT defined.
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Bruce Mayer, PE Chabot College Mathematics
Example Example Find Domain Find Domain
Find the domain of each function.
a. f x log3 2 x b. f x log3
x 2
x 1
Solution a.The Domain of a logarithmic function must be positive, that is,
2 x 0
2 x
Thus The domain of f is (−∞, 2).
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Bruce Mayer, PE Chabot College Mathematics
Example Example Find Domain Find Domain
Find the domain of each function.
b. f x log3
x 2
x 1
Solution b.The Domain of a logarithmic function must be positive, that is,
Need to Avoid Negative-Logs AND Division by Zero
x 2
x 1 0
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Bruce Mayer, PE Chabot College Mathematics
Example Example Find Domain Find Domain
Soln b. (cont.) b. f x log3
x 2
x 1
Set numerator = 0 & denominator = 0
Construct a SIGN CHART
x − 2 = 0 x + 1 = 0x = 2 x = −1
The domain of f is (−∞, −1)U(2, ∞).
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Bruce Mayer, PE Chabot College Mathematics
Properties of Exponential and Properties of Exponential and Logarithmic FunctionsLogarithmic Functions
Exponential Function f (x) = ax
Logarithmic Function f (x) = loga x
Domain (0, ∞) Range (–∞, ∞)
Domain (–∞, ∞) Range (0, ∞)
x-intercept is 1 No y-intercept
y-intercept is 1 No x-intercept
x-axis (y = 0) is the horizontal asymptote
y-axis (x = 0) is the vertical asymptote
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Bruce Mayer, PE Chabot College Mathematics
Properties of Exponential and Properties of Exponential and Logarithmic FunctionsLogarithmic Functions
Exponential Function f (x) = ax
Logarithmic Function f (x) = loga x
Is one-to-one, that is, logau = logav if and only if u = v
Is one-to-one , that is, au = av if and only if u = v
Increasing if a > 1 Decreasing if 0 < a < 1
Increasing if a > 1 Decreasing if 0 < a < 1
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Bruce Mayer, PE Chabot College Mathematics
Graphs of Logarithmic FcnsGraphs of Logarithmic Fcns
f (x) = loga x (0 < a < 1)f (x) = loga x (a > 1)
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Bruce Mayer, PE Chabot College Mathematics
Graph Logs by TranslationGraph Logs by Translation
Start with the graph of f(x) = log3x and use Translation Transformations to sketch the graph of each function
a. f x log3 x 2 b. f x log3 x 1
Also State the DOMAIN and RANGE and the VERTICAL ASYMPTOTE for the graph of each function
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Bruce Mayer, PE Chabot College Mathematics
Graph Logs by TranslationGraph Logs by Translation
Solutiona. f x log3 x 2
• Shift UP 2
• Domain (0, ∞)
• Range (−∞, ∞)
• Vertical asymptote x = 0
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Bruce Mayer, PE Chabot College Mathematics
Graph Logs by TranslationGraph Logs by Translation
Solution
• Shift RIGHT 1
• Domain (1, ∞)
• Range (−∞, ∞)
• Vertical asymptote x = 1
b. f x log3 x 1
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Bruce Mayer, PE Chabot College Mathematics
WhiteBoard WorkWhiteBoard Work
Problems From §9.3 Exercise Set• 8, 18, 26, 38, 48
Logs &ExponentialsAre InverseFunctions
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Bruce Mayer, PE Chabot College Mathematics
All Done for TodayAll Done for Today
Inventorof
Logarithms
Born: 1550 in Merchiston Castle, Edinburgh, Scotland
Died: 4 April 1617 in Edinburgh, Scotland
John Napier
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Bruce Mayer, PE Chabot College Mathematics
Bruce Mayer, PELicensed Electrical & Mechanical Engineer
Chabot Mathematics
AppendiAppendixx
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srsrsr 22