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Given a logarithm with the form log b M , use the change-of-base formula to rewrite it as a quotient of logs with any positive base n , where n 1.

  1. Determine the new base n , remembering that the common log, log ( x ) , has base 10, and the natural log, ln ( x ) , has base e .
  2. Rewrite the log as a quotient using the change-of-base formula
    • The numerator of the quotient will be a logarithm with base n and argument M .
    • The denominator of the quotient will be a logarithm with base n and argument b .

Changing logarithmic expressions to expressions involving only natural logs

Change log 5 3 to a quotient of natural logarithms.

Because we will be expressing log 5 3 as a quotient of natural logarithms, the new base, n = e .

We rewrite the log as a quotient using the change-of-base formula. The numerator of the quotient will be the natural log with argument 3. The denominator of the quotient will be the natural log with argument 5.

log b M = ln M ln b log 5 3 = ln 3 ln 5
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Change log 0.5 8 to a quotient of natural logarithms.

ln 8 ln 0.5

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Can we change common logarithms to natural logarithms?

Yes. Remember that log 9 means log 10 9 . So, log 9 = ln 9 ln 10 .

Using the change-of-base formula with a calculator

Evaluate log 2 ( 10 ) using the change-of-base formula with a calculator.

According to the change-of-base formula, we can rewrite the log base 2 as a logarithm of any other base. Since our calculators can evaluate the natural log, we might choose to use the natural logarithm, which is the log base e .

log 2 10 = ln 10 ln 2 Apply the change of base formula using base  e . 3.3219 Use a calculator to evaluate to 4 decimal places .
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Evaluate log 5 ( 100 ) using the change-of-base formula.

ln 100 ln 5 4.6051 1.6094 = 2.861

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Access these online resources for additional instruction and practice with laws of logarithms.

Key equations

The Product Rule for Logarithms log b ( M N ) = log b ( M ) + log b ( N )
The Quotient Rule for Logarithms log b ( M N ) = log b M log b N
The Power Rule for Logarithms log b ( M n ) = n log b M
The Change-of-Base Formula log b M = log n M log n b           n > 0 , n 1 , b 1

Key concepts

  • We can use the product rule of logarithms to rewrite the log of a product as a sum of logarithms. See [link] .
  • We can use the quotient rule of logarithms to rewrite the log of a quotient as a difference of logarithms. See [link] .
  • We can use the power rule for logarithms to rewrite the log of a power as the product of the exponent and the log of its base. See [link] , [link] , and [link] .
  • We can use the product rule, the quotient rule, and the power rule together to combine or expand a logarithm with a complex input. See [link] , [link] , and [link] .
  • The rules of logarithms can also be used to condense sums, differences, and products with the same base as a single logarithm. See [link] , [link] , [link] , and [link] .
  • We can convert a logarithm with any base to a quotient of logarithms with any other base using the change-of-base formula. See [link] .
  • The change-of-base formula is often used to rewrite a logarithm with a base other than 10 and e as the quotient of natural or common logs. That way a calculator can be used to evaluate. See [link] .

Section exercises

Verbal

How does the power rule for logarithms help when solving logarithms with the form log b ( x n ) ?

Any root expression can be rewritten as an expression with a rational exponent so that the power rule can be applied, making the logarithm easier to calculate. Thus, log b ( x 1 n ) = 1 n log b ( x ) .

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What does the change-of-base formula do? Why is it useful when using a calculator?

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Algebraic

For the following exercises, expand each logarithm as much as possible. Rewrite each expression as a sum, difference, or product of logs.

log b ( 7 x 2 y )

log b ( 2 ) + log b ( 7 ) + log b ( x ) + log b ( y )

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log b ( 13 17 )

log b ( 13 ) log b ( 17 )

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ln ( 1 4 k )

k ln ( 4 )

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For the following exercises, condense to a single logarithm if possible.

ln ( 7 ) + ln ( x ) + ln ( y )

ln ( 7 x y )

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log 3 ( 2 ) + log 3 ( a ) + log 3 ( 11 ) + log 3 ( b )

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log b ( 28 ) log b ( 7 )

log b ( 4 )

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ln ( a ) ln ( d ) ln ( c )

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log b ( 1 7 )

log b ( 7 )

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For the following exercises, use the properties of logarithms to expand each logarithm as much as possible. Rewrite each expression as a sum, difference, or product of logs.

log ( x 15 y 13 z 19 )

15 log ( x ) + 13 log ( y ) 19 log ( z )

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ln ( a −2 b −4 c 5 )

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log ( x 3 y 4 )

3 2 log ( x ) 2 log ( y )

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log ( x 2 y 3 x 2 y 5 3 )

8 3 log ( x ) + 14 3 log ( y )

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For the following exercises, condense each expression to a single logarithm using the properties of logarithms.

log ( 2 x 4 ) + log ( 3 x 5 )

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ln ( 6 x 9 ) ln ( 3 x 2 )

ln ( 2 x 7 )

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2 log ( x ) + 3 log ( x + 1 )

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log ( x ) 1 2 log ( y ) + 3 log ( z )

log ( x z 3 y )

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4 log 7 ( c ) + log 7 ( a ) 3 + log 7 ( b ) 3

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For the following exercises, rewrite each expression as an equivalent ratio of logs using the indicated base.

log 7 ( 15 ) to base e

log 7 ( 15 ) = ln ( 15 ) ln ( 7 )

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log 14 ( 55.875 ) to base 10

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For the following exercises, suppose log 5 ( 6 ) = a and log 5 ( 11 ) = b . Use the change-of-base formula along with properties of logarithms to rewrite each expression in terms of a and b . Show the steps for solving.

log 11 ( 5 )

log 11 ( 5 ) = log 5 ( 5 ) log 5 ( 11 ) = 1 b

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log 11 ( 6 11 )

log 11 ( 6 11 ) = log 5 ( 6 11 ) log 5 ( 11 ) = log 5 ( 6 ) log 5 ( 11 ) log 5 ( 11 ) = a b b = a b 1

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Numeric

For the following exercises, use properties of logarithms to evaluate without using a calculator.

log 3 ( 1 9 ) 3 log 3 ( 3 )

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6 log 8 ( 2 ) + log 8 ( 64 ) 3 log 8 ( 4 )

3

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2 log 9 ( 3 ) 4 log 9 ( 3 ) + log 9 ( 1 729 )

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For the following exercises, use the change-of-base formula to evaluate each expression as a quotient of natural logs. Use a calculator to approximate each to five decimal places.

log 1 2 ( 4.7 )

2.23266

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Extensions

Use the product rule for logarithms to find all x values such that log 12 ( 2 x + 6 ) + log 12 ( x + 2 ) = 2. Show the steps for solving.

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Use the quotient rule for logarithms to find all x values such that log 6 ( x + 2 ) log 6 ( x 3 ) = 1. Show the steps for solving.

x = 4 ; By the quotient rule: log 6 ( x + 2 ) log 6 ( x 3 ) = log 6 ( x + 2 x 3 ) = 1.

Rewriting as an exponential equation and solving for x :

6 1 = x + 2 x 3 0 = x + 2 x 3 6 0 = x + 2 x 3 6 ( x 3 ) ( x 3 ) 0 = x + 2 6 x + 18 x 3 0 = x 4 x 3 x = 4

Checking, we find that log 6 ( 4 + 2 ) log 6 ( 4 3 ) = log 6 ( 6 ) log 6 ( 1 ) is defined, so x = 4.

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Can the power property of logarithms be derived from the power property of exponents using the equation b x = m ? If not, explain why. If so, show the derivation.

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Prove that log b ( n ) = 1 log n ( b ) for any positive integers b > 1 and n > 1.

Let b and n be positive integers greater than 1. Then, by the change-of-base formula, log b ( n ) = log n ( n ) log n ( b ) = 1 log n ( b ) .

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Does log 81 ( 2401 ) = log 3 ( 7 ) ? Verify the claim algebraically.

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Questions & Answers

how did you get 1640
Noor Reply
If auger is pair are the roots of equation x2+5x-3=0
Peter Reply
Wayne and Dennis like to ride the bike path from Riverside Park to the beach. Dennis’s speed is seven miles per hour faster than Wayne’s speed, so it takes Wayne 2 hours to ride to the beach while it takes Dennis 1.5 hours for the ride. Find the speed of both bikers.
MATTHEW Reply
420
Sharon
from theory: distance [miles] = speed [mph] × time [hours] info #1 speed_Dennis × 1.5 = speed_Wayne × 2 => speed_Wayne = 0.75 × speed_Dennis (i) info #2 speed_Dennis = speed_Wayne + 7 [mph] (ii) use (i) in (ii) => [...] speed_Dennis = 28 mph speed_Wayne = 21 mph
George
Let W be Wayne's speed in miles per hour and D be Dennis's speed in miles per hour. We know that W + 7 = D and W * 2 = D * 1.5. Substituting the first equation into the second: W * 2 = (W + 7) * 1.5 W * 2 = W * 1.5 + 7 * 1.5 0.5 * W = 7 * 1.5 W = 7 * 3 or 21 W is 21 D = W + 7 D = 21 + 7 D = 28
Salma
Devon is 32 32​​ years older than his son, Milan. The sum of both their ages is 54 54​. Using the variables d d​ and m m​ to represent the ages of Devon and Milan, respectively, write a system of equations to describe this situation. Enter the equations below, separated by a comma.
Aaron Reply
find product (-6m+6) ( 3m²+4m-3)
SIMRAN Reply
-42m²+60m-18
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-24m+3+3mÁ^2
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Bajemah
-6m(3mA²+4m-3)+6(3mA²+4m-3) =-18m²A²-24m²+18m+18mA²+24m-18 Rearrange like items -18m²A²-24m²+42m+18A²-18
Salma
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Jovelyn Reply
x=3-2y
Salma
y=x+3/2
Salma
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Enock
given that (7x-5):(2+4x)=8:7find the value of x
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3x-12y=18
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A meditation garden is in the shape of a right triangle, with one leg 7 feet. The length of the hypotenuse is one more than the length of one of the other legs. Find the lengths of the hypotenuse and the other leg
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The Jones family took a 15 mile canoe ride down the Indian River in three hours. After lunch, the return trip back up the river took five hours. Find the rate, in mph, of the canoe in still water and the rate of the current.
cameron Reply
Shakir works at a computer store. His weekly pay will be either a fixed amount, $925, or $500 plus 12% of his total sales. How much should his total sales be for his variable pay option to exceed the fixed amount of $925.
mahnoor Reply
I'm guessing, but it's somewhere around $4335.00 I think
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12% of sales will need to exceed 925 - 500, or 425 to exceed fixed amount option. What amount of sales does that equal? 425 ÷ (12÷100) = 3541.67. So the answer is sales greater than 3541.67. Check: Sales = 3542 Commission 12%=425.04 Pay = 500 + 425.04 = 925.04. 925.04 > 925.00
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When traveling to Great Britain, Bethany exchanged $602 US dollars into £515 British pounds. How many pounds did she receive for each US dollar?
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Jazmine trained for 3 hours on Saturday. She ran 8 miles and then biked 24 miles. Her biking speed is 4 mph faster than her running speed. What is her running speed?
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Sheirtina
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Source:  OpenStax, College algebra. OpenStax CNX. Feb 06, 2015 Download for free at https://legacy.cnx.org/content/col11759/1.3
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