How to Solve Exponential Equations Requiring Logarithms

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Some exponential equations cannot be rewritten with a common base. Use logarithms to bring the variable down from the exponent.

Key idea

If \(b^u=N\), then \(u=\frac{ln N}{ln b}\). Isolate the exponential expression first, then take logs.

Solving Exponential Equations Requiring Logarithms

Think of this lesson as more than a rule to memorize. Solving Exponential Equations Requiring Logarithms is about rewriting exponent questions and solving growth equations. A strong student does not rush to the first formula on the page; they pause, identify the structure of the problem, and then choose the tool that matches that structure. That pause is what prevents most mistakes.

A logarithm answers an exponent question: \(\log_b a=c\) means \(b^c=a\). This translation is the safest first step.

Here is the teacher way to approach the topic. First, read the problem slowly and underline the information that is actually given. Next, name the target: are you finding a value, simplifying an expression, comparing two quantities, solving for a variable, or interpreting a graph? Once the target is clear, the calculation becomes much less mysterious because every step has a job.

Clear clutter such as parentheses or fractions.
Collect like terms.
Undo operations in reverse order.
Substitute the answer back or test a point.

A helpful habit is to say what each number represents before using it. For example, if a number is a denominator, a radius, a slope, a common difference, or a coefficient, it should not be treated like an ordinary loose number. Its role tells you where it belongs in the formula. This is especially important on ACT-style questions because many wrong answer choices come from using the right numbers in the wrong places.

Another good habit is to keep the work organized vertically. Write one clean line for substitution, one line for simplifying, and one line for the final answer. If the problem has units, keep the units attached. If the problem has signs, exponents, or parentheses, copy them carefully from one line to the next. Most errors in this topic are not caused by a hard idea; they are caused by dropping a negative sign, combining unlike terms, using the wrong denominator, or skipping a check.

When you finish, ask a quick reasonableness question. Should the answer be positive or negative? Should it be larger or smaller than the original number? Does it fit the graph, table, shape, or equation? Can you plug it back into the original problem? This final check turns the lesson from a procedure into understanding.

On a test, the goal is not to write the longest solution. The goal is to write enough clear work that you can see the structure, avoid traps, and recover quickly if one line goes wrong. Practice the examples below with that mindset: identify the type of problem, choose the matching rule, show the substitution, simplify carefully, and check the answer before moving on.

Free printable Worksheets

Exercises for Solving Exponential Equations Requiring Logarithms

1) Find \(x\): \(2^x=13\)

2) Find \(x\): \(5^x=72\)

3) Find \(x\): \(3^{x+2}=50\)

4) Find \(x\): \(7^{2x-1}=100\)

5) Find \(x\): \(e^x=18\)

6) Find \(x\): \(4e^{x-3}=28\)

7) Find \(x\): \(6^{x/2}=41\)

8) Find \(x\): \(9^{3x+1}=500\)

9) Find \(x\): \(2^{x+4}-5=70\)

10) A model is \(P=12(1.08)^t\). Find \(t\) when \(P=30\).

11) Find \(r\): \(3^{2r-5}=19\)

12) Find \(x\): \(10-4(0.6)^x=7\)

13) Find \(x\): \(5e^{2x+1}=60\)

14) Find \(k\): \(8^{k-2}+11=40\)

15) Find \(x\): \(16^{x+1}=5^{2x-1}\)

16) Find \(x\): \(2^x=7^{x-3}\)

17) Find \(t\): \(150(0.92)^t=90\)

18) Find \(x\): \(4^{x+2}=3(2^x)\)

19) Find \(x\): \(e^x+3e^{-x}=4\).

20) Find the positive solution of \(9^x-4(3^x)-45=0\).

Answers

1)Find \(x\): \(2^x=13\)
\(\color{red}{Isolate the exponential part: \(2^x=13\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{x=\frac{ln 13}{ln 2} \approx 3.7004.}\)

2)Find \(x\): \(5^x=72\)
\(\color{red}{Isolate the exponential part: \(5^x=72\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{x=\frac{ln 72}{ln 5} \approx 2.6572.}\)

3)Find \(x\): \(3^{x+2}=50\)
\(\color{red}{Isolate the exponential part: \(3^{x+2}=50\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{x=\frac{ln 50}{ln 3}-2 \approx 1.5609.}\)

4)Find \(x\): \(7^{2x-1}=100\)
\(\color{red}{Isolate the exponential part: \(7^{2x-1}=100\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{x=\frac{\frac{ln 100}{ln 7}+1}{2} \approx 1.6833.}\)

5)Find \(x\): \(e^x=18\)
\(\color{red}{Isolate the exponential part: \(e^x=18\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{x=ln 18 \approx 2.8904.}\)

6)Find \(x\): \(4e^{x-3}=28\)
\(\color{red}{Isolate the exponential part: \(e^{x-3}=7\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{x=3+ln 7 \approx 4.9459.}\)

7)Find \(x\): \(6^{x/2}=41\)
\(\color{red}{Isolate the exponential part: \(6^{x/2}=41\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{x=2\frac{ln 41}{ln 6} \approx 4.1452.}\)

8)Find \(x\): \(9^{3x+1}=500\)
\(\color{red}{Isolate the exponential part: \(9^{3x+1}=500\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{x=\frac{\frac{ln 500}{ln 9}-1}{3} \approx 0.6095.}\)

9)Find \(x\): \(2^{x+4}-5=70\)
\(\color{red}{Isolate the exponential part: \(2^{x+4}=75\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{x=\frac{ln 75}{ln 2}-4 \approx 2.2288.}\)

10)A model is \(P=12(1.08)^t\). Find \(t\) when \(P=30\).
\(\color{red}{Isolate the exponential part: \((1.08)^t=2.5\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{t=\frac{ln 2.5}{ln 1.08} \approx 11.9059.}\)

11)Find \(r\): \(3^{2r-5}=19\)
\(\color{red}{Isolate the exponential part: \(3^{2r-5}=19\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{r=\frac{\frac{ln 19}{ln 3}+5}{2} \approx 3.8401.}\)

12)Find \(x\): \(10-4(0.6)^x=7\)
\(\color{red}{Isolate the exponential part: \((0.6)^x=0.75\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{x=\frac{ln 0.75}{ln 0.6} \approx 0.5632.}\)

13)Find \(x\): \(5e^{2x+1}=60\)
\(\color{red}{Isolate the exponential part: \(e^{2x+1}=12\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{x=\frac{ln 12-1}{2} \approx 0.7425.}\)

14)Find \(k\): \(8^{k-2}+11=40\)
\(\color{red}{Isolate the exponential part: \(8^{k-2}=29\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{k=2+\frac{ln 29}{ln 8} \approx 3.6193.}\)

15)Find \(x\): \(16^{x+1}=5^{2x-1}\)
\(\color{red}{Isolate the exponential part: \((x+1)ln 16=(2x-1)ln 5\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{x=\frac{-ln 5-ln 16}{ln 16-2ln 5} \approx 9.8189.}\)

16)Find \(x\): \(2^x=7^{x-3}\)
\(\color{red}{Isolate the exponential part: \(x ln 2=(x-3)ln 7\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{x=\frac{-3ln 7}{ln 2-ln 7} \approx 4.6599.}\)

17)Find \(t\): \(150(0.92)^t=90\)
\(\color{red}{Isolate the exponential part: \((0.92)^t=0.6\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{t=\frac{ln 0.6}{ln 0.92} \approx 6.1264.}\)

18)Find \(x\): \(4^{x+2}=3(2^x)\)
\(\color{red}{Isolate the exponential part: \(2^{2x+4}=3(2^x)\).}\)
\(\color{red}{Take natural logs and use ln(a^u)=u ln(a).}\)
\(\color{red}{x=\frac{ln 3}{ln 2}-4 \approx -2.415.}\)

19)Find \(x\): \(e^x+3e^{-x}=4\).
\(\color{red}{Let y=e^x, so y+3/y=4.}\)
\(\color{red}{Then y^2-4y+3=0, so y=1 or y=3.}\)
\(\color{red}{Thus x=0 or x=ln 3.}\)

20)Find the positive solution of \(9^x-4(3^x)-45=0\).
\(\color{red}{Let y=3^x, so y^2-4y-45=0.}\)
\(\color{red}{(y-9)(y+5)=0; y must be positive, so y=9.}\)
\(\color{red}{Therefore 3^x=9 and x=2.}\)

How to Solve Exponential Equations Requiring Logarithms