Logarithmic Equation

CAT 2021 Slot 2 · QA · Hard · Logarithms

If 5log101+x+4log101x=log1011x25 - \log_{10}\sqrt{1 + x} + 4\log_{10}\sqrt{1 - x} = \log_{10}\frac{1}{\sqrt{1 - x^2}}, then 100x100x equals

Answer

99

Explanation

Simplify the log terms: 512log10(1+x)+2log10(1x)=12log10(1x2)5 - \frac{1}{2}\log_{10}(1+x) + 2\log_{10}(1-x) = -\frac{1}{2}\log_{10}(1-x^2) Notice that log10(1x2)=log10(1+x)+log10(1x)\log_{10}(1-x^2) = \log_{10}(1+x) + \log_{10}(1-x). So, 512log10(1+x)+2log10(1x)=12log10(1+x)12log10(1x)5 - \frac{1}{2}\log_{10}(1+x) + 2\log_{10}(1-x) = -\frac{1}{2}\log_{10}(1+x) - \frac{1}{2}\log_{10}(1-x)

Cancel 12log10(1+x)-\frac{1}{2}\log_{10}(1+x) from both sides: 5+2log10(1x)=12log10(1x)5 + 2\log_{10}(1-x) = -\frac{1}{2}\log_{10}(1-x) 5=52log10(1x)5 = -\frac{5}{2}\log_{10}(1-x) log10(1x)=2\log_{10}(1-x) = -2 1x=102=0.011 - x = 10^{-2} = 0.01 x=0.99x = 0.99 Therefore, 100x=99100x = 99.

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