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Exponential Equations (Cambridge O Level Additional Maths)
Revision Note
Solving Exponential Equations
What are exponential equations?
- An exponential equation is an equation where the unknown is a power
- In simple cases the solution can be spotted without the use of a calculator
- For example,
- In more complicated cases the laws of logarithms should be used to solve exponential equations
- The change of base law can be used to solve some exponential equations without a calculator
- For example,
How do we use logarithms to solve exponential equations?
- An exponential equation can be solved by taking logarithms of both sides
- The laws of indices may be needed to rewrite the equation first
- The laws of logarithms can then be used to solve the equation
- ln (loge) is often used
- The answer is often written in terms of ln
- A question my ask you to give your answer in a particular form
- Follow these steps to solve exponential equations
- STEP 1: Take logarithms of both sides
- STEP 2: Use the laws of logarithms to remove the powers
- STEP 3: Rearrange to isolate x
- STEP 4: Use logarithms to solve for x
What about hidden quadratics?
- Look for hidden squared terms that could be changed to form a quadratic
- In particular look out for terms such as
- 4x = (22)x = 22x = (2x)2
- e 2x = (e2)x = (ex)2
- In particular look out for terms such as
Examiner Tip
- Always check which form the question asks you to give your answer in, this can help you decide how to solve it
- If the question requires an exact value you may need to leave your answer as a logarithm
Worked example
Solve the equation . Give your answer correct to three significant figures.
'Spot' the hidden quadratic by noticing that .
Rewrite the first term as a power of 2.
Rewrite the middle terms using the laws of indices: If
USing a substitution can make this easier to solve.
Let
Factorise.
Solve to find u and substitute 2x back in.
Solve the exponential equation 2x = 3 by taking logarithms of both sides.
Bring the power down using the law of logs .
Rearrange and solve.
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