Example

Example: Evaluating Logarithms by Converting to Exponential Form

To evaluate a logarithm without a calculator, such as log525\log_5 25, the expression can be converted into an exponential equation. First, set the expression equal to a variable, yielding log525=x\log_5 25 = x. Next, change this to exponential form, which is 5x=255^x = 25. Then, rewrite the constant so both sides share the same base; since 25=5225 = 5^2, the equation becomes 5x=525^x = 5^2. With the same base, the exponents must be equal, meaning x=2x = 2. Therefore, log525=2\log_5 25 = 2. This method also applies to fractional results, such as setting log93=x\log_9 3 = x to get 9x=39^x = 3, which simplifies to (32)x=31(3^2)^x = 3^1 or 32x=313^{2x} = 3^1, yielding 2x=12x = 1 and x=12x = \frac{1}{2}. It also applies to negative results, such as setting log2116=x\log_2 \frac{1}{16} = x to get 2x=1162^x = \frac{1}{16}, which rewrites as 2x=1242^x = \frac{1}{2^4} and then 2x=242^x = 2^{-4}, yielding x=4x = -4.

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Updated 2026-05-25

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Ch.10 Exponential and Logarithmic Functions - Intermediate Algebra @ OpenStax

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