1. Precalculus Essentials
Fifth Edition
Chapter 3
Exponential and Logarithmic Functions
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2. 3.4 Exponential and Logarithmic Equations

3. Objectives Use like bases to solve exponential equations.
Use logarithms to solve exponential equations.
Use the definition of a logarithm to solve logarithmic equations.
Use the one-to-one property of logarithms to solve logarithmic equations.
Solve applied problems involving exponential and logarithmic equations.

4. Solving Exponential Equations by Expressing Each Side as a Power of the Same Base
Set M = N.
Solve for the variable.

5. Example 1: Solving Exponential Equations
Solution:
The solution set is {−12}.
The solution set is {3}.

6. Using Logarithms to Solve Exponential Equations
Isolate the exponential expression.
Take the common logarithm on both sides of the equation for base 10. Take the natural logarithm on both sides of the equation for bases other than 10.
Simplify using one of the following properties:
Solve for the variable.

7. Example 2: Solving Exponential Equations
Solution:

8. Example 3: Solving Exponential Equations
Solution:
The solution set is {log 8000}.

9. Example 4: Solving an Exponential Equation
Solution:

10. Using the Definition of a Logarithm to Solve Logarithmic Equations
Use the definition of a logarithm to rewrite the equation in exponential form:
Solve for the variable.
Check proposed solutions in the original equation. Include in the solution set only values for which M > 0.

11. Example 1: Solving Logarithmic Equations
Solution:
True. The solution set is {12}.

12. Example 2: Solving Logarithmic Equations
Solution:

13. Example 3: Solving Logarithmic Equations (1 of 2)
Solution:

14. Example 3: Solving Logarithmic Equations (2 of 2)
We are solving log x + log(x − 3) = 1 and have found two possible solutions, x = 5 and x = −2.
The number −2 does not check. The domain of logarithmic functions consists of positive numbers.
True The solution set is {5}.

15. Using the One-to-One Property of Logarithms to Solve Logarithmic Equations
Solve for the variable.
Check proposed solutions in the original equation. Include in the solution set only values for which M > 0 and N > 0.

16. Example: Solving a Logarithmic Equation (1 of 3)
Solution:

17. Example: Solving a Logarithmic Equation (2 of 3)
We are solving ln(x − 3) = ln(7x − 23) − ln(x +1) and have found two possible solutions, x = 5 and x = 4.
True. 5 is a solution for the equation.

18. Example: Solving a Logarithmic Equation (3 of 3)
The equation is true for both x = 5 and x = 4. The solution set is {4, 5}.

19. Example 1: Application
How long, to the nearest tenth of a year, will it take $1000 to grow to $3600 at 8% annual interest compounded quarterly?
After approximately 16.2 years, $1000 will grow to an accumulated value of $3600.

20. Example 2: Application
The percentage of adult height attained by a girl who is x years old can be modeled by f(x) = log(x − 4) where x represents the girl’s age (from 5 to 15) and f(x) represents the percentage of her adult height. At what age has a girl attained 97% of her adult height?
At approximately age 14, a girl has attained 97% of her adult height.