College Algebra
Fifth Edition
James Stewart  Lothar Redlin

Saleem Watson
5
Exponential and
Logarithmic
Functions
5.3
Laws of Logarithms
Laws of Logarithms
In this section, we study properties
of logarithms.
• These properties give logarithmic functions
a wide range of applications—as we will see
in Section 5.5.
Laws of Logarithms
Laws of Logarithms
Since logarithms are exponents,
the Laws of Exponents give rise to
the Laws of Logarithms.
Laws of Logarithms
Let a be a positive number, with a ≠ 1.
Let A, B, and C be any real numbers
with A > 0 and B > 0.
Law 1
loga ( AB )  loga A  loga B
• The logarithm of a product of numbers
is the sum of the logarithms of the
numbers.
Law 2
 A
loga    loga A  loga B
B
• The logarithm of a quotient of numbers
is the difference of the logarithms of
the numbers.
Law 3
   C log
C
loga A
a
A
• The logarithm of a power of a number
is the exponent times the logarithm of
the number.
Laws of Logarithms—Proof
We make use of the property
loga ax = x
from Section 5.2.
Law 1—Proof
Let loga A = u and loga B = v.
• When written in exponential form,
these equations become: au  A and av  B
• Thus,
loga ( AB )  loga (au av )  loga (au v )
 u v
 loga A  loga B
Law 2—Proof
Using Law 1, we have:
 A  
 A
loga A  loga   B   loga    loga B
B
 B  
• Thus,
 A
loga    loga A  loga B
B
Law 3—Proof
Let loga A = u.
Then, au = A.
• So,
C
u
loga A  loga a
 
 
C
 
 loga a
uC
 uC  C loga A
E.g. 1—Using the Laws to Evaluate Expressions
Evaluate each expression.
(a) log4 2  log4 32
(b) log2 80  log2 5
(c)  31 log8
E.g. 1—Evaluating Expressions
Example (a)
log4 2  log4 32
 log4 (2  32)
Law 1
 log4 64
3
Because 64 = 4
3
E.g. 1—Evaluating Expressions
Example (b)
log2 80  log2 5
 80 
 log2  
 5 
Law 2
 log2 16
4
Because 16 = 2
4
E.g. 1—Evaluating Expressions
Example (c)
 31 log8
 log81/ 3
Law 3
 log  21 
Property of negative exponents
 0.301
Calculator
Expanding and Combining
Logarithmic Expressions
Expanding Logarithmic Expressions
The laws of logarithms allow us to write
the logarithm of a product or a quotient as
the sum or difference of logarithms.
• This process—called expanding a logarithmic
expression—is illustrated in the next example.
E.g. 2—Expanding Logarithmic Expressions
Use the Laws of Logarithms to expand
each expression.
(a) log2 (6 x )

(b) log5 x 3 y 6
 ab 
(c) ln  3 
 c

E.g. 2—Expanding Log Expressions Examples (a) & (b)
log2 (6 x )  log2 6  log2 x

3
log5 x y
6
  log
5
x  log5 y
3
Law 1
6
 3 log5 x  6 log5 y
Law 1
Law 3
E.g. 2—Expanding Log Expressions
 ab 
3
ln  3   ln(ab )  ln c
 c
1/ 3
 ln a  ln b  ln c
 ln a  ln b  ln c
1
3
Law 2
Law 1
Law 3
Combining Logarithmic Expressions
The laws of logarithms also allow us to
reverse the process of expanding done
in Example 2.
• That is, we can write sums and differences
of logarithms as a single logarithm.
• This process—called combining logarithmic
expressions—is illustrated in the next example.
E.g. 3—Combining Logarithmic Expressions
Combine 3 log x + ½ log(x + 1) into
a single logarithm.
3log x  21 log( x  1)
 log x  log( x  1)
3

 log x
1/ 2
3
 x  1
1/ 2

(Law 3)
(Law 1)
E.g. 4—Combining Logarithmic Expressions
Combine 3 ln s + ½ ln t – 4 ln(t2 + 1)
into a single logarithm.
3ln s  ln t  4ln(t  1)
2
1
2
 ln s 3  ln t 1/ 2  ln(t 2  1)4
(Law 3)
 ln(s 3t 1/ 2 )  ln(t 2  1)4
(Law 1)
 3

s t 

 ln
 t2 1 4 


(Law 2)


Warning
Although the Laws of Logarithms tell us how
to compute the logarithm of a product or
a quotient, there is no corresponding rule for
the logarithm of a sum or a difference.
Warning
For instance,

loga  x  y   loga x  loga y
• In fact, we know that the right side
is equal to loga(xy).
Warning
Also, don’t improperly simplify quotients
or powers of logarithms.
• For instance,

log6
6
 log  
log2
2
log2 x 
 3log x
3
2
Applying Laws of Logarithms
Logarithmic functions are used to
model a variety of situations involving
human behavior.
• One such behavior is how quickly
we forget things we have learned.
Applying Laws of Logarithms
For example, if you learn algebra at a certain
performance level (say 90% on a test) and
then don’t use algebra for a while, how much
will you retain after a week, a month, or a
year?
• Hermann Ebbinghaus (1850–1909) studied this
phenomenon and formulated the law described
in the next example.
E.g. 5—The Law of Forgetting
Ebbinghaus’ Law of Forgetting states that,
if a task is learned at a performance level P0,
then, after a time interval t, the performance
level P satisfies
log P = log P0 – c log(t + 1)
where:
• c is a constant that depends on the type of task.
• t is measured in months.
E.g. 5—The Law of Forgetting
(a) Solve for P.
(b) If your score on a history test is 90,
what score would you expect to get
on a similar test after two months?
After a year?
•
Assume c = 0.2.
E.g. 5—The Law of Forgetting
Example (a)
We first combine the right-hand side.
log P  log P0  c log(t  1)
log P  log P0  log(t  1)
c
P0
log P  log
(t  1)c
P0
P
(t  1)c
(Law 3)
(Law 2)
(Because log is one-to-one)
E.g. 5—The Law of Forgetting
Example (b)
Here, P0 = 90, c = 0.2, and t is measured in
months.
In two months: t  2
In one year:
and
t  12 and
P
P
90
 2  1
0.2
 72
90
12  1
0.2
• Your expected scores after two months and
one year are 72 and 54, respectively.
 54
The Law of Forgetting
Forgetting what we’ve learned
depends logarithmically on how long
ago we learned it.
Change of Base
Change of Base
For some purposes, we find it useful to
change from logarithms in one base to
logarithms in another base.
• Suppose we are given loga x
and want to find logb x.
Change of Base
Let
y = logb x
• We write this in exponential form and take
the logarithm, with base a, of each side—as
follows.
Change of Base
by  x
loga  b
Exponential form
y
  log
a
x
y loga b  loga x
Taking loga of each side
Law 3
loga x
y
loga b
• This proves the following formula.
Change of Base Formula
loga x
logb x 
loga b
• In particular, if we put x = a, then loga a = 1,
and this formula becomes:
1
logb a 
loga b
Change of Base
We can now evaluate a logarithm to any
base by:
1. Using the Change of Base Formula to express
the logarithm in terms of common logarithms or
natural logarithms.
2. Using a calculator.
E.g. 6—Evaluating Logarithms
Use the Change of Base Formula and
common or natural logarithms to evaluate
each logarithm, correct to five decimal places.
(a) log8 5
(b) log9 20
E.g. 6—Evaluating Logarithms
Example (a)
We use the Change of Base Formula with
b = 8 and a = 10:
log10 5
log8 5 
log10 8
 0.77398
E.g. 6—Evaluating Logarithms
Example (b)
We use the Change of Base Formula with
b = 9 and a = e:
ln20
log9 20 
ln9
 1.36342
E.g. 7—Graphing a Logarithmic Function
Use a graphing calculator to graph
f(x) = log6 x
• Calculators don’t have a key for log6.
• So, we use the Change of Base Formula
to write:
ln x
f ( x )  log6 x 
ln 6
E.g. 7—Graphing a Logarithmic Function
Since calculators do have an LN key,
we can enter this new form of the function
and graph it.