Long Log

Algebra Level 2

log x x ( log x x ) = x 2 2 \large \log_{\frac{\sqrt{x}}{x}}{(\log_{x}{\sqrt{x}})}=\frac{\frac{x}{2}}{2}

What is x x if it's a positive integer?


The answer is 4.

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1 solution

Chew-Seong Cheong
Mar 20, 2018

log x x ( log x x ) = x 2 2 log x 1 2 ( log x x 1 2 ) = x 4 log x 1 2 ( 1 2 log x x ) = x 4 log x 1 2 ( 1 2 ) = x 4 log x 1 2 log x x 1 2 = x 4 2 log x 2 = x 4 x = 8 log x 2 x x = 2 8 = 4 4 x = 4 \begin{aligned} \log_{\frac {\sqrt x}x} \left(\log_x \sqrt x\right) & = \frac {\frac x2}2 \\ \log_{x^{-\frac 12}} \left(\log_x x^\frac 12\right) & = \frac x4 \\ \log_{x^{-\frac 12}} \left(\frac 12 \log_x x\right) & = \frac x4 \\ \log_{x^{-\frac 12}} \left(\frac 12\right) & = \frac x4 \\ \frac {\log_x \frac 12}{\log_x x^{-\frac 12}} & = \frac x4 \\ 2\log_x 2 & = \frac x4 \\ \implies x & = 8 \log_x 2 \\ x^x & = 2^8 = 4^4 \\ \implies x & = \boxed{4} \end{aligned}

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