Logarithm Problem

Algebra Level 3

log 10 x = ( ln x ) 2 \log_{10} x = (\ln x)^2 has two solutions. One of them is x = 1 x = 1 . What is the other solution?


The answer is 1.543873444.

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1 solution

First start with changing the base of log x: l o g ( x ) = l n ( x ) l n ( 10 ) log(x) = \frac{ln(x)} {ln(10)} . The equation can be rewritten as: ( l n ( x ) ) 2 l n ( x ) l n ( 10 ) = l n ( x ) ( l n ( x ) 1 l n ( 10 ) ) = 0 (ln(x))^2 - \frac{ln(x)} {ln(10)} = ln(x) (ln(x) - \frac{1} {ln(10)}) =0 . So ln (x) =0, which gives x = 1. Or l n ( x ) 1 l n ( 10 ) = 0 ln(x) - \frac{1} {ln(10)} = 0 . This leads to x = e 1 l n ( 10 ) x = e^{ \frac{1} {ln(10)}} .

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