2.5 Exponential and Logarithmic Equations

Learning Objectives

In this section, you will:

  • Use logarithms to solve exponential equations.
  • Use the definition of a logarithm to solve logarithmic equations.
  • Solve applied problems involving exponential and logarithmic equations.
Seven rabbits in front of a brick building.

Figure 1. Wild rabbits in Australia. The rabbit population grew so quickly in Australia that the event became known as the “rabbit plague.” (credit: Richard Taylor, Flickr)

In 1859, an Australian landowner named Thomas Austin released 24 rabbits into the wild for hunting. Because Australia had few predators and ample food, the rabbit population exploded. In fewer than ten years, the rabbit population numbered in the millions.

Uncontrolled population growth, as in the wild rabbits in Australia, can be modeled with exponential functions. Equations resulting from those exponential functions can be solved to analyze and make predictions about exponential growth. In this section, we will learn techniques for solving exponential equations.

Solving Exponential Equations Using Logarithms

Many times we need to solve equations of the form [latex]n=ab^x[/latex] where [latex]n[/latex] is a real number.  To do this we will divide both sides by [latex]a[/latex] to get [latex]\frac{n}{a}=b^x[/latex] and then take the logarithm of both sides giving the equation [latex]\mathrm{log}\left(\frac{n}{a}\right)=\mathrm{log}\left(b^x\right).[/latex] Next, we use the power rule for logarithms to get [latex]\mathrm{log}\left(\frac{n}{a}\right)=x\mathrm{log}\left(b\right)[/latex].  Finally, divide both sides by [latex]\mathrm{log}\left(b\right)[/latex]  to get [latex]x=\frac{\mathrm{log}\left(\frac{n}{a}\right)}{\mathrm{log}\left(b\right)}[/latex].  Note that any base for the logarithm can be used but base 10 and base [latex]e[/latex] are most commonly used.

How To

Given an exponential equation in the form [latex]n=ab^x[/latex], solve for the unknown.

  1. Divide both sides by [latex]a[/latex] or the initial condition.
  2. Apply the logarithm of both sides of the equation.
    • If one of the terms in the equation has base 10, use the common logarithm.
    • If none of the terms in the equation has base 10, use the natural logarithm.
  3. Use the rules of logarithms to solve for the unknown.

Example 1:  Solving a Basic Exponential Equation

Solve [latex]5=3\left(2\right)^x.[/latex]

Try It #1

Solve [latex]7=15\left(4\right)^x.[/latex]

Example 2:  Solving an Exponential Equation with an Algebraic Expression in the Exponent

Solve [latex]15=3\left(0.5\right)^{x+1}.[/latex]

Try It #2

Solve [latex]15=2\left(7\right)^{x^2+1}.[/latex]

Equations Containing e

One common type of exponential equations uses base [latex]e.[/latex] This constant occurs again and again in nature, in mathematics, in science, in engineering, and in finance. When we have an equation with a base [latex]e[/latex] on either side, we can use the natural logarithm to solve it.

How To

Given an equation of the form [latex]y=a{e}^{kt}\text{,}[/latex] solve for [latex]t.[/latex]

  1. Divide both sides of the equation by [latex]a.[/latex]
  2. Apply the natural logarithm of both sides of the equation.
  3. Divide both sides of the equation by [latex]k.[/latex]

Example 3:  Solve an Equation with Continuous Growth

Solve [latex]100=20{e}^{2t}.[/latex]

Try it #3

Solve [latex]3{e}^{0.5t}=11.[/latex]

Q&A

Do all exponential equations have a solution? If not, how can we tell if there is a solution during the problem-solving process?

No. Recall that the range of an exponential function is always positive. While solving the equation, we may obtain an expression that is undefined. 

Example 4:  Solving an Equation with Positive and Negative Powers

Solve [latex]{3}^{x+1}=-2.[/latex]

Try it #4

Solve [latex]{2}^{x}=-100.[/latex]

Sometimes there are exponential functions on both sides of the equation such as [latex]ab^x=cd^x[/latex].  The process is similar to solving [latex]n=ab^x[/latex] but we will need to be careful to use the product rule of logarithms before applying the power rule of logarithms.

How To

Given an equation with exponential expression on each side, solve for the unknown.

  1. Divide both sides by one of the initial conditions.
  2. Apply the logarithm of both sides of the equation.
    • If one of the terms in the equation has base 10, use the common logarithm.
    • If none of the terms in the equation has base 10, use the natural logarithm.
  3. Use the product and power rules of logarithms and then solve for the unknown.

Example 5:  Solving an Equation Containing Powers of Different Bases

Solve [latex]6\left(5\right)^{x+2}=2\left(4\right)^{x}.[/latex]

Try it #5

Solve [latex]{2}^{x}={3}^{x+1}.[/latex]

Q&A

Does every equation of the form [latex]y=A{e}^{kt}[/latex] have a solution?

No. There is a solution when [latex]k\ne 0,[/latex] and when [latex]y[/latex] and [latex]A[/latex] are either both 0 or neither 0, and they have the same sign. An example of an equation with this form that has no solution is [latex]2=-3{e}^{t}.[/latex]

Example 6:  Solving an Equation That Requires Algebra First

Solve

  1. [latex]4{e}^{2x}+5=12.[/latex]
  2. [latex]{e}^{2t}-3=-4{e}^{2t}[/latex]

Try it #6

Solve [latex]3+{e}^{2t}=7{e}^{2t}.[/latex]

Extraneous Solutions

Sometimes the methods used to solve an equation introduce an extraneous solution, which is a solution that is correct algebraically but does not satisfy the conditions of the original equation. One such situation arises in solving when the logarithm is taken on both sides of the equation. In such cases, remember that the argument of the logarithm must be positive. If the number we are evaluating in a logarithm function is negative, there is no output.

These extraneous solutions frequently occur when the exponential function is a quadratic form. Recall that quadratic equations can be solved by factoring and setting each factor equal to zero, or by the quadratic equation.  W will look for the pattern of the quadratic and then choose which technique can most easily be used.

Example 7:  Solving Exponential Functions in Quadratic Form

Solve [latex]{e}^{2x}-{e}^{x}=56.[/latex]

Try it #7

Solve [latex]{e}^{2x}={e}^{x}+2.[/latex]

Using the Definition of a Logarithm to Solve Logarithmic Equations

We have already seen that the logarithmic equation [latex]{\mathrm{log}}_{b}\left(x\right)=y[/latex] is equivalent to the exponential equation [latex]{b}^{y}=x,[/latex] for [latex]x>0.[/latex] We can use this fact, along with the rules of logarithms, to solve logarithmic equations where the argument is an algebraic expression.

Example 8:  Solving a Logarithmic Equation

Solve the equation [latex]{\mathrm{log}}_{2}\left(2\right)+{\mathrm{log}}_{2}\left(3x-5\right)=3.[/latex]

Example 9:  Using Algebra to Solve a Logarithmic Equation

Solve [latex]2\mathrm{ln}\left(x\right)+3=7.[/latex]

Try it #8

Solve [latex]6+\mathrm{ln}\left(x\right)=10.[/latex]

Example 10:  Using Algebra Before and After Using the Definition of the Natural Logarithm

Solve [latex]2\mathrm{ln}\left(6x\right)=7.[/latex]

Try it #9

Solve [latex]2\mathrm{ln}\left(x+1\right)=10.[/latex]

Example 11:  Using a Graph to Understand the Solution to a Logarithmic Equation

Solve [latex]\mathrm{ln}\left(x\right)=3.[/latex]

Try it #10

Use a graphing calculator to estimate the approximate solution to the logarithmic equation [latex]{2}^{x}=1000[/latex] to 2 decimal places.

Solving Applied Problems Using Exponential Equations

In previous sections, we learned the properties and rules for both exponential and logarithmic functions. We have seen that any exponential function can be written as a logarithmic function and vice versa. We have used exponents to solve logarithmic equations and logarithms to solve exponential equations. We are now ready to combine our skills to solve equations that model real-world situations, whether the unknown is in an exponent or in the argument of a logarithm.

Example 12:  Real World application; Depreciation

In 2018, a car was purchased for $32,000 and depreciates 20% per year.  When will the car be worth $8,000?

Example 13:  Real World Application; BLood Alcohol Content

A person’s blood alcohol content (BAC) is a measure of how much alcohol is in the bloodstream.  When a person stops drinking, over time the BAC will decay exponentially.  For a particular individual, the formula [latex]f\left(t\right)=0.1e^{-0.0067t}[/latex] models their BAC over time, t, measured in minutes.  When will their BAC be 0.04?

Example 14:  Real World Application; Tuition

College A is charging $40,000 tuition in 2019 and it is increasing at a continuous rate of 7% per year.  College B charges $45,000 in 2019, but it is increasing at 4% per year.  When will college A cost more than college B?

Try It #11

A town’s population is 14,000 people in 2017 and is increasing at a rate of 2.1% each year.  When will the town’s population reach 18,000 people?

Conversions Between Continuous and Noncontinuous Growth Rates

Recall that there are two possible formulas that can be used to represent an exponential function: [latex]f(x)=ab^x=a{\left(b\right)}^x[/latex] for noncontinuous growth and [latex]f(x)=ae^{kx}=a{\left(e^k\right)}^x[/latex] for continuous growth.  When comparing the two forms, we see that [latex]b=e^k.[/latex]  Further, since [latex]b=1+r[/latex] where [latex]r[/latex] is the noncontinuous growth rate, we have that [latex]1+r=e^k.[/latex]

Example 15:  Continuous Growth to Noncontinuous Growth

Given a continuous growth rate of 12%, find the noncontinuous rate.

Example 16:  Noncontinuous Growth to Continuous Growth

Given a noncontinuous growth rate of 15%, find the continuous rate.

Try It #12

a.  Given a continuous decreasing rate of 5%, find the noncontinuous rate.

b.  Given a noncontinuous increasing rate of 7%, find the continuous rate.

Solving Logarithmic Applications

Exponential growth and decay often involve very large or very small numbers. It is common to use a logarithmic scale when measurements result in extremely large values or extremely small values.  The Richter Scale, pH and decibels are examples of such scales.

To describe these numbers, we often use orders of magnitude. The order of magnitude is the power of ten, when the number is expressed in scientific notation, with one digit to the left of the decimal. For example, the distance to the nearest star, Proxima Centauri, measured in kilometers, is 40,113,497,200,000 kilometers. Expressed in scientific notation, this is 4.01134972 x 1013 . So, we could describe this number as having order of magnitude of 13.

The magnitude (size) of an earthquake is measured on a scale known as the Richter Scale. The Richter Scale is a base-ten logarithmic scale. In other words, an earthquake of magnitude 6 is not twice as great as an earthquake of magnitude 3.  It is 106−3 = 10= 1,000 times as great!

The Richter scale strength of an earthquake, M, is given by [latex]M=\mathrm{log}\left(\frac{W}{W_0}\right),[/latex] where [latex]W[/latex] is the strength of the seismic waves of an earthquake and [latex]W_0[/latex] is the strength of normally occurring earthquakes.  Minor earthquakes occur regularly allowing [latex]W_0[/latex] to be determined.

Example 17:  The Richter Scale

An earthquake Richter Scale strength of 5 is considered a moderate strength earthquake.  How much stronger was the 2010 magnitude 5.5 earthquake that occurred between Ontario and Quebec compared to a standard earthquake?

In chemistry, pH is used as a measure of the acidity or alkalinity of a substance. The pH scale runs from 0 to 14. Substances with a pH less than 7 are considered acidic, and substances with a pH greater than 7 are said to be alkaline. Our bodies, for instance, must maintain a pH close to 7.35 in order for enzymes to work properly. To get a feel for what is acidic and what is alkaline, consider the following pH levels of some common substances:

  • Battery acid: 0.8
  • Stomach acid: 2.7
  • Orange juice: 3.3
  • Pure water: 7 (at 25° C)
  • Human blood: 7.35
  • Fresh coconut: 7.8
  • Sodium hydroxide (lye): 14

To determine whether a solution is acidic or alkaline, we find its pH, which is a measure of the number of active positive hydrogen ions in the solution. The pH is defined by the following formula, where [latex]\left[H^+\right][/latex] is the concentration of hydrogen ions in the solution measured in moles per liter.

[latex]pH=-\mathrm{log}\left(\left[H^+\right]\right)=\mathrm{log}\left(\frac{1}{\left[H^+\right]}\right)[/latex]

Example 18:  pH

If the concentration of hydrogen ions in a liquid is doubled, what is the effect on pH?

Example 19:  pH

Orange juice has a pH of approximately 3.3.  Determine the hydrogen ion concentration.

A decibel (dB) is a measure of how loud a sound is when compared to a reference value.  A commonly used reference value is the sound intensity of the softest sound a human can typically hear; usually that of a child.  We will call this value [latex]I_0.[/latex]  Since there is a very wide range of sounds that humans can hear, the logarithmic scale is used. The formula for decibels is

Sound level in decibels [latex]=10\mathrm{log}\left(\frac{I}{I_0}\right),[/latex]

where [latex]I[/latex] is the sound intensity of the sound being measured.

Example 20:  Decibels

A vacuum cleaner sound level measures at 75dB and a balloon popping measures 125dB.   A balloon popping is how many times more intense than the sound intensity of the vacuum cleaner?

 

Access these online resources for additional instruction and practice with exponential and logarithmic equations.

Key Concepts

  • An exponential equation can be solved by taking the logarithm of each side.
  • We can solve exponential equations with base [latex]e,[/latex] by applying the natural logarithm of both sides because exponential and logarithmic functions are inverses of each other.
  • After solving an exponential equation, check each solution in the original equation to find and eliminate any extraneous solutions.
  • When given an equation of the form [latex]{\mathrm{log}}_{b}\left(S\right)=c,[/latex] where [latex]S[/latex] is an algebraic expression, we can use the definition of a logarithm to rewrite the equation as the equivalent exponential equation [latex]{b}^{c}=S,[/latex] and solve for the unknown.
  • We can also use graphing to solve equations with the form [latex]{\mathrm{log}}_{b}\left(S\right)=c.[/latex] We graph both equations [latex]y={\mathrm{log}}_{b}\left(S\right)[/latex] and [latex]y=c[/latex] on the same coordinate plane and identify the solution as the x-value of the intersecting point.
  • Combining the skills learned in this and previous sections, we can solve equations that model real world situations, whether the unknown is in an exponent or in the argument of a logarithm.

Glossary

extraneous solution
a solution introduced while solving an equation that does not satisfy the conditions of the original equation