# Chapter 5 � Exponential Functions & Logarithms by J74oS5w

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```									L5.5 Logarithmic Functions

a logarithm is an exponent
a logarithm is an exponent
a logarithm is an exponent
a logarithm is an exponent
L5.5 Warm up
Find the value of x
•   10x = 100,000     x=5
•   2x = 1/64         x = –6
•   ex = e1/x         x = 1/x → x2 = 1; x = ±1
•   4x = –1           no soln
•   4x = 2048         22x = 211
2x = 11
x = 11/2

When you can equate bases, exponents can be equated.
If you can not equate bases, you need logarithms…
Logarithmic Functions
f(x) = 3x
•   Sketch graphs of f(x) = 3x and f-1(x)          f-1(x) = log3 x

x    f(x)                                   x     f-1(x)
-2   1/9                                    1/9    -2
-1   1/3              (0, 1)                1/3    -1
0     1                                     1       0
1     3                        (1, 0)       3       1
2     9                                     9       2

The inverse of the exponential function y = bx is x = by
and x = by is the logarithmic function y = logb x.

x = by    ↔   y = logb x
Logarithmic Functions
f(x) = bx                                     f-1(x) = logb x

Notice that the bases, b, are
(0, 1)                   the same for the inverses and
that b > 0, b ≠ 1.
(1, 0)

Exponential functions (with a = 1):               Logarithmic functions:
Are continuous and one-to-one                     Are continuous and one-to-one
Domain: {x | x is Real}                           Domain: {x | x > 0}
Range: {y | y > 0 }                               Range: {y | y is Real }
Contain the point (0, 1)                          Contain the point (1, 0)
y = 0 is a horizontal asymptote                   x = 0 is a vertical asymptote

Because exponential and logarithmic functions are inverses of each
other, they swap their domains and ranges.
The restriction on the domain of logs is very important. Why can’t it be negative?
Try log(-5) or ln(-5) in your calculator. What happens?
Logs are just Exponents!
x = by    ↔   y = logb x
Base is 4.
Examples: 64 = 43 ↔ 3 = log4 64           Exponent is 3.
Result is 64.
Base is 2.
32 =   25   ↔ 5 = log2 32     Exponent is 5.
Result is 32.
Base is 10.
100 =   102    ↔ 2 = log10 100   Exponent is 2.
Result is 100.

You should be able to convert between exponential and
logarithmic forms:
log2 16 = 4         24 = 16
log10 1000 = 3      103 = 1000
62 = 36             log6 36 = 2
34 = 81             log3 81 = 4
Finding Logs
Two approaches:

1) If the input to the log function is a power of the base, you
can find the log without a calculator.

2) If the input to the log function is not a power of the base,
you need a calculator to find the log.
Finding Logs (when input is a power of the base)
These can be done without a calculator.
Solve:
“What exponent do I raise 10 to to get 10,000?”
x = log10 10,000       x = 4 because 104 = 10,000
“What exponent do I raise 5 to to get 1/125?”
x = log5 1/125         x = -3 because 5-3 = 1/125

Do these:                             Note: log6 63 = 3 and 10log10 4 = 4

1. x = log6 216         x=3                            same bases

2. x = log7 1/49        x = -2                 logb bx = x and blog b x = x
3. x = log10 0.0001     x = -4             Logs & exponentials are inverses!

4. x = log4 –4          No solution. Domain > 0!
(There is no exponent that I can raise 4 to to get -4)
Finding Logs (when input is not a power of the base)
These require a calculator.
Calculators have logs for 2 bases built in: 10 and e*.            log() on Ti89
log10 is the Common Log. log x without a base means log10      x Is “hotkey” ◊7
log 75 means “what do I raise 10 to to get 75”?
log 75 ≈ 1.875 b/c 101.875 ≈ 75                 It’s between 1 & 2.
Loge is the Natural Log. ln x means loge x.                        Why?
ln 32 means “what do I raise e to to get 32”?
ln 32 ≈ 3.466 b/c e3.466 ≈ 32

Do these (to 2 decimal places):
You should check
2. x = log 0.03           x = –1.52
* We will learn how to use the
3. x = ln 104             x = 4.64            calculator for bases other than
4. x = ln 0.8             x = – .22           10 and e later.
Homework (due Tuesday)
pp 194-195 #1 – 9 odd, 10, 11 – 17 odd, 28, 29, 34
(mostly odds so you can check your homework).

You are encouraged to read Lesson 5.5 if you are
confused about logs and/or you wait to do your
homework until Monday night.

Remember:
a logarithm is an exponent
a logarithm is an exponent
a logarithm is an exponent
a logarithm is an exponent
L5.5 Checkpoint after day 1

• A logarithm is just ________________

• Find f-1(x) if f(x) = 7x

• What is the domain of f-1(x)? Why? What is the range? Why?

• Convert between log and exp’l forms:
1. log 500 = x
2. ex = e5
3. ex = 200
4. log 1000 = x

• Solve the above problems (which ones need a calculator?)
L5.5 Summary (so far):

•   A logarithms is just an exponent
•   The inverse of f(x) = bx is f-1(x) = logb x
•   The domain of a log function is {x | x > 0} Why?
•   You can convert between logarithmic and exponential forms
x = by   ↔   y = logb x

• To find a logarithm:
• If the input is a power of the base, just return the exponent
• If the input is not a power of the base, use your calculator
• common log: log x means log10 x
• natural log: ln x means loge x
• we’ll learn later how to find logs of other bases

• Next: Applications of logs
• Logs are used when the range of information is huge
Wrap Up (day 1) or Warm Up (day 2)
≈8
0.9031
10
Class exercises p194 #1-9
#3. Given log 8 ≈ 0.9031. [How could you check this?]
Find log 80 and log 0.08 without a calculator.

log 8 = 0.9031 → 100.9031 = 8     10???? = 80 ← log 80
10*100.9031 = 8*10
101.9031 = 80
log 80 = 1.9031

log 8 = 0.9031 → 100.9031 = 8    10???? = 0.08 ← log 0.08
10-2*100.9031 = 8*10-2
10-1.0969 = 0.08
log 0.08 = -1.0969
Decibels: An Application of Logs
Decibels (dB) is a measure of the loudness of sound.
The decibel scale is an example of a logarithmic scale*.
Given a sound intensity level, I,
I
dB = 10 log , where I0 is the intensity of a barely audible sound.
I0
[Page 192 has a table with intensity levels for common sounds.]
* There are many others:
Examples:                                                  brightness, acidity, &
1. Average car @ 70kmh, I = 106.8I0. dB?                   earthquake magnitude.

I        106.8 I 0
10 log  10 log            (10) log106.8  68dB
I0          I0                               I person whispering: 15dB
2 people             18dB
2. Whisper, I = 101.5I0. dB of 2 people whispering?
I          2 101.5 I 0
10 log  10 log                 (10) log(2 101.5 )  18dB
I0              I0
You do not have to memorize these defintions.
Richter Scale: An Application of Logs
•   Earthquakes are measured on the Richter scale which is a
logarithmic scale.
•   The Magnitude, M, of an earthquake on the Richter scale
is as follows:          2    E
M  log
3    E0
where E is the energy released by the earthquake
measured in joules and E0 = 104.4 joules is the energy
released by a very small reference earthquake.

1. The 1906 San Francisco earthquake released
approximately 5.96 X 1016 joules of energy. What was the
magnitude of this earthquake on the Richter scale?
2    E    2    5.96 X 1016
M  log 4.4  log       4.4
 8.25
3   10    3       10
Richter Scale: An Application of Logs
2. Generally, an earthquake requires a magnitude over 5.6
on the Richter scale to inflict serious damage. How
many times more powerful than this was the great 1906
Columbia earthquake, which registered a magnitude of
8.6 on the Richter scale?
1906 Columbia Earthquake                      Earthquake with magnitude 5.6
2     E                                         2     E
log     8.6                                    log     5.6
3     E0                                        3     E0
E                                               E
log        4.4
 12.9                         log               8.4
4.4
10                                              10
E
E
 10
12.9                                          108.4
10 4.4                                          104.4
E  1017.3 joules                                  E  1012.8 joules

1017.3
12.8
 32,000 times more powerful
10
Logs are Exponents!
If a log is an expression with multiple terms, you can use
the rules of exponents to simplify
Ex: If ln y = 4x + 2 show that y ≈ 7.4(54.6)x
y = e4x+2          convert to exponential form
y = e4x · e2       use rules of exponents
y = e2 · (e4)x     use rules of exponents
y ≈ 7.4(54.6)x     estimate e2 and e4

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