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College Algebra Tutorial 45: Exponential Equations

Learning Objectives After completing this tutorial, you should be able to: 1. Solve exponential equations.

Introduction In this tutorial I will walk you through how to solve equations that have exponential expressions. In these equations, you will notice that the variable that we are solving for is in the exponent. We are use to seeing the variable in the base. We will using inverse operations like we do in linear equations, the inverse operation we will be using here is logarithms. If you need a review on the definition of log functions, feel free to go to Tutorial 43: Logarithmic Functions. If you need a review on log properties, feel free to go to Tutorial 44: Logarithmic Properties. I think you are ready to get started.

Tutorial

Solving Exponential Equations, where x is in the exponent, BUT the bases DO NOT MATCH.

Step 1: Isolate the exponential expression. Get your exponential expression on one side everything outside of the exponential expression on the other side of your equation.

Step 2: Take the natural log of both sides. The inverse operation of an exponential expression is a log. Make sure that you do the same thing to both sides of your equation to keep them equal to each other.

Step 3: Use the properties of logs to pull the x out of the exponent. If you need a review on log properties, feel free to go to Tutorial 44: Logarithmic Properties. I think you are ready to get started.

Step 4: Solve for x. Now that the variable is out of the exponent, solve for the variable using inverse operations to complete the problem.

Special Note: The only way we can get that variable out of the exponent, when the bases don’t match up, is to use logs. The 3rd step allows us to do this. When solving an equation, it doesn’t matter what you do to the equation as long as you do the same thing to both sides - this keeps both sides equal. Also, the reason we take the natural log of both sides is because we have the natural log key on the calculator - so we would be able to find a value of it in the end.

Example 1: Solve the exponential equation

. Round your

answer to two decimal places.

Step 1: Isolate the exponential expression. This is already done for us in this problem.

Step 2: Take the natural log of both sides.

*Take the natural log of BOTH sides

Step 3: Use the properties of logs to pull the x out of the exponent.

*Use the power rule

Step 4: Solve for x.

*Inverse of mult. by 3lne is to divide by 3lne

*Use the calculator to find ln 50 *lne is 1

Example 2: Solve the exponential equation

. Round your

answer to two decimal places.

Step 1: Isolate the exponential expression.

*Inverse of mult. by 5 is to divide by 5 *Exponential expression isolated

Step 2: Take the natural log of both sides.

*Take the natural log of BOTH sides

Step 3: Use the properties of logs to pull the x out of the exponent.

*Use the power rule

Step 4: Solve for x.

*Inverse of mult. by ln 10 is to divide by ln 10

*Inverse of add 1 is sub. 1

*Use the calculator to find ln 2.4 and ln 10

Example 3: Solve the exponential equation answer to two decimal places.

. Round your

Step 1: Isolate the exponential expression.

*Inverse of add 4 is sub. 4 *Exponential expression isolated

Step 2: Take the natural log of both sides.

*Take the natural log of BOTH sides

Step 3: Use the properties of logs to pull the x out of the exponent.

*Use the power rule

Step 4: Solve for x.

*Inverse of mult. by .2ln 2 is to divide by .2ln 2

*Use the calculator to find ln 21 and ln 2

Example 4: Solve the exponential equation your answer to two decimal places.

. Round

Step 1: Isolate the exponential expression. Notice how we have two exponential terms that have different exponents. We wouldn’t be able to isolate both. We will have to figure out another way to rewrite it so we can continue with the steps. Note how we have a trinomial and that e to the 2x is e to the x squared. This means it is quadratic in from. So we can factor it just like a trinomial of the form .

*Factor the trinomial of the form

.

*Set the 1st factor = 0 *Isolate the exponential expression

*Set the 2nd factor = 0 *Isolate the exponential expression

Note that since e is a positive base, no matter what the exponent is on x, this exponential expression CANNOT equal -2. So there is only one equation that we can solve

.

Step 2: Take the natural log of both sides.

*Take the natural log of BOTH sides

Step 3: Use the properties of logs to pull the x out of the exponent.

*Use the power rule

Step 4: Solve for x.

*Inverse of mult. by lne is to divide by lne

*Use the calculator to find ln 4 *lne = 1

Practice Problems These are practice problems to help bring you to the next level. It will allow you to check and see if you have an understanding of these types of problems. Math works just like anything else, if you want to get good at it, then you need to practice it. Even the best athletes and musicians had help along the way and lots of practice, practice, practice, to get good at their sport or instrument. In fact there is no such thing as too much practice. To get the most out of these, you should work the problem out on your own and then check your answer by clicking on the link for the answer/discussion for that problem. At the link you will find the answer as well as any steps that went into finding that answer.

Practice Problems 1a - 1c: Solve the given exponential equation. Round your answer to two decimal places.

1a. (answer/discussion to 1a)

1b. (answer/discussion to 1b)

1c. (answer/discussion to 1c)

Need Extra Help on These Topics?

The following are webpages that can assist you in the topics that were covered on this page: http://www.purplemath.com/modules/solvexpo.htm This webpage helps you with exponential equations. http://www.sosmath.com/algebra/solve/solve7/s72/s72.html This webpage gives an example of solving an exponential equation. http://www.sosmath.com/algebra/solve/solve7/s73/s73.html This webpage gives an example of solving an exponential equation.

Go to Get Help Outside the Classroom found in Tutorial 1: How to Succeed in a Math Class for some more suggestions.

(Back to the College Algebra Homepage) All contents copyright (C) 2002, WTAMU and Kim Peppard. All rights reserved. Last revised on November 4, 2002 by Kim Peppard.