Solve logarithmic equations

This lesson will discuss two ways to solve logarithmic equations. The method you use depends on the type of logarithmic equation you are trying to solve. Keep reading and you will learn how to solve the one below in a heartbeat!

Solve logarithmic equations

The first type you will learn to solve has the following format

logb X = Y

To solve it, you just have to write it in exponential form as X = bY

Example #1

log (2x + 1) = 3

log10 (2x + 1) = 3

After writing it in exponential form we get

2x + 1 = 103

2x + 1 = 1000

2x = 999

x = 499.5

Check x = 499.5

log10 (2 x 499.5 + 1)  = log10 (1000) = 3 since 103 = 1000

Sometimes when you solve logarithmic equations, you need to put all the logarithms on one side of the equation. Example #2 shows how to do this.

Example #2

log5 (30x - 10) - 2 = log5 (x + 6)

You can solve this one the same way you solved example #1.

Start by putting all logarithms on one side of the equation.

log5 (30x - 10) - log5 (x + 6) - 2 = log5 (x + 6) - log5 (x + 6)

log5 (30x - 10) - log5 (x + 6) -2 = 0

log5 (30x - 10) - log5 (x + 6) -2 + 2 = 0 + 2

log5 (30x - 10) - log5 (x + 6)  =  2

log5 [ (30x - 10) / (x + 6) ] = 2

Now that it has the format that we want, we can just write it in exponential form.

30x - 10 / x + 6 = 52

30x - 10 / x + 6 = 25       ( After multiplying both sides by x + 6 )

30x - 10 = 25 (x + 6)

30x - 10 = 25x + 150

30x - 25x = 150 + 10

5x = 160

x = 32

Check x = 32

log5 (30x - 10) - 2 = log5 (x + 6)

log5 (30 x 32 - 10) - 2 = log5 (32 + 6)

log5 (960 - 10) - 2 = log5 (32 + 6)

log5 (950) - 2 = log5 (38)

4.26 - 2 = 2.26

2.26 = 2.26

x = 32 is a good answer.

Now, you will learn how to solve logarithmic equations that have the following format

logb X = logb Y

To solve it, you just have to see that logb X = logb Y if and only if X = Y

More examples showing how to solve logarithmic equations using logarithmic properties.


Example #3

log6 (2x - 4) + log6 (4) = log6 40

log6 4(2x - 4)  = log6 40

log6 8x - 16  = log6 40

8x - 16 = 40

8x = 40 + 16

8x = 56

x = 7

Check x = 7

log6 (2 x 7 - 4) + log6 (4) = log6 40

log6 (14 - 4) + log6 (4) = log6 40

log6 (10) + log6 (4) = log6 40

log6 (10 x 4) = log6 40

log6 (40) = log6 40

x is 7 is a good answer.

Example #4

log3 x + log3 (x + 3)  = log3 (2x + 6)

log3 x(x + 3)  = log3 (2x + 6)

log3 (x2 + 3x)  = log3 (2x + 6)

x2 + 3x  = 2x + 6


x2 + 3x - 2x - 6 = 0

x2 + x - 6 = 0

(x - 2) (x + 3) = 0

x = 2 and x  = -3

Check x = 2

log3 2 + log3 (2 + 3)  = log3 (2 x 2 + 6)

log3 2 + log3 (5)  = log3 (4 + 6)

 log3 2(5)  = log3 (4 + 6)

log3 (10)  = log3 (10)

x  = 2  is a good answer.

Check x = -3

log3 -3 + log3 (-3 + 3)  = log3 (2 x -3 + 6)

log3 -3 + log3 0 = log3 0

Since 
log3 -3 does not exist, x  = -3 cannot be an answer.

Recent Articles

  1. Quadratic Formula: Easy To Follow Steps

    Jan 26, 23 11:44 AM

    Quadratic formula
    Learn about the quadratic formula, the discriminant, important definitions related to the formula, and applications.

    Read More

  2. Area Formula - List of Important Formulas

    Jan 25, 23 05:54 AM

    Frequently used area formulas
    What is the area formula for a two-dimensional figure? Here is a list of the ones that you must know!

    Read More

Tough algebra word problems

100 Tough Algebra Word Problems.

If you can solve these problems with no help, you must be a genius!

Math quizzes

 Recommended

Math vocabulary quizzes