Download Properties of Logarithms

Properties of Logarithms
Since the exponential and logarithmic functions with base a are inverse functions, 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 > 0, B > 0,
and C be any real numbers.
Law
Description
1. log a (AB) = log a A + log a B
⎛ A⎞
2. log a ⎜ ⎟ = log a A – log a B
⎝B⎠
3. log a (AC) = C log a A
Example 1:
The logarithm of a product of numbers is
the sum of the logarithms of the numbers.
The logarithm of a quotient of numbers is the
difference of the logarithms of the numbers.
The logarithm of a power of a number is the
exponent times the logarithm of the number.
Use the Laws of Logarithms to rewrite the expression in a form with no
logarithm of a product, quotient, or power.
⎛ 5 x3 ⎞
(a) log 3 ⎜ 2 ⎟
⎝ y ⎠
(b) ln 5 x 2 ( z + 1)
Solution (a):
⎛ 5 x3 ⎞
log 3 ⎜ 2 ⎟ = log 3 ( 5 x3 ) − log 3 y 2
⎝ y ⎠
= log 3 5 + log 3 x 3 − log 3 y 2
= log 3 5 + 3log 3 x − 2 log 3 y
Law 2
Law 1
Law 3
By: Crystal Hull
Example 1 (Continued)
Solution (b):
ln 5 x 2 ( z + 1) = ln ( x 2 ( z + 1) )
Example 2:
1
5
= 15 ln ( x 2 ( z + 1) )
Law 3
= 15 ⎡⎣ ln x 2 + ln ( z + 1) ⎤⎦
Law 1
= 15 ln x 2 + 15 ln ( z + 1)
Distribution
= 15 ( 2 ln x ) + 15 ln ( z + 1)
Law 3
= 52 ln x + 15 ln ( z + 1)
Multiplication
Rewrite the expression as a single logarithm.
(a) 3log 4 5 + log 4 10 − log 4 7
(b) log x − 2 log y + 12 log ( 9 z 2 )
Solution (a):
3log 4 5 + log 4 10 − log 4 7 = log 4 53 + log 4 10 − log 4 7
= log 4 125 + log 4 10 − log 4 7
Law 3
Because 53 = 125
= log 4 (125 ⋅10 ) − log 4 7
Law 1
= log 4 (1250 ) − log 4 7
Simplification
⎛ 1250 ⎞
= log 4 ⎜
⎟
⎝ 7 ⎠
Law 2
By: Crystal Hull
Example 2 (Continued):
Solution (b):
log x − 2 log y + 12 log ( 9 z 2 ) = log x − log y 2 + log ( 9 z 2 )
1
2
Law 3
= log x − log y 2 + log ( 3z )
Simplification
= log
Law 2
( ) + log (3z )
= log ⎡( ) ( 3 z ) ⎤
⎣
⎦
= log ( )
x
y2
x
y2
Law 1
3 zx
y2
Example 3:
Simplification
Evaluate the expression
(a) log 2 5 16
(b) log 3 189 − log 3 7
Solution (a):
1
log 2 5 16 = log 2 16 5
= 15 log 2 16
Law 3
= 15 log 2 2
Because 16 = 24
4
= 15 ( 4 )
Property 3 of Logarithms
(Section 6.2)
=
4
5
Multiplication
Solution (b):
log 3 189 − log 3 7 = log 3 189
7
Law 2
= log 3 27
Simplification
= log 3 33
Because 27 = 33
=3
Property 3 of Logarithms
(Section 6.2)
By: Crystal Hull
Change of Base:
A calculator can be used to approximate the values of common logarithms (base 10) or
natural logarithms (base e). However, sometimes we need to use logarithms to other
bases. The following rule is used to convert logarithms from one base to another.
Change of Base Formula:
log b x =
Example 4:
log a x
log a b
Use the Change of Base Formula and a calculator to evaluate the
logarithm, correct to six decimal places. Use either natural or common
logarithms.
(a) log 6 17
(b) log 5 2.33
Solution (a):
The Change of Base Formula says
log b x =
log a x
.
log a b
Thus, if we let the new base a = 10
log 6 17 =
log10 17
≈ 1.581246
log10 6
Solution (b):
Again we will use the Change of Base Formula. This time we will let the
new base be a = e.
log 5 2.33 =
ln 2.33
≈ 0.525568
ln 5
By: Crystal Hull
Example 5:
Simplify:
( log8 12 )( log12 7 )
Solution:
Using the Change of Base Formula with the new base a = 10 :
log8 12 =
log12
log 8
and
log12 7 =
log 7
log12
Thus,
⎛ log12 ⎞⎛ log 7 ⎞
⎟⎜
⎟
⎝ log 8 ⎠⎝ log12 ⎠
( log8 12 )( log12 7 ) = ⎜
log 7
log 8
= log8 7
=
Simplification
Change of Base Formula
By: Crystal Hull