Logarithmic Equation

Algebra Level 2

Given that

log 2 ( log 8 x ) = log 8 ( log 2 x ) , \log_2(\log_8x)=\log_8(\log_2x),

find the value of ( log 2 x ) 2 (\log_2x)^2 .


The answer is 27.

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5 solutions

Using the Properties of Logarithms , this equation can be written as

log 2 ( ( 1 3 ) log 2 x ) = ( 1 3 ) log 2 ( log 2 x ) \log_{2}((\frac{1}{3})\log_{2} x) = (\frac{1}{3})\log_{2}(\log_{2} x)

( 1 3 ) log 2 x = ( log 2 x ) 1 3 \Longrightarrow (\frac{1}{3})\log_{2} x = (\log_{2} x)^{\frac{1}{3}}

( log 2 x ) 2 3 = 3 log 2 x = 3 3 \Longrightarrow (\log_{2} x)^{\frac{2}{3}} = 3 \Longrightarrow \log_{2} x = 3\sqrt{3} ,

the square of which is 9 3 = 27 9 \cdot 3 = \boxed{27} .

Victor Loh
Nov 17, 2014

log 2 ( log 8 x ) = log 8 ( log 2 x ) \log_2(\log_8x)=\log_8(\log_2x)

log 2 ( log 2 x 3 ) = log 2 ( log 2 x ) 3 \implies\log_2\left(\frac{\log_2x}{3}\right)=\frac{\log_2(\log_2x)}{3}

log 2 ( log 2 x 3 ) = log 2 ( ( log 2 x ) 1 / 3 ) \implies\log_2\left(\frac{\log_2x}{3}\right)=\log_2\left((\log_2x)^{1/3}\right)

log 2 x 3 = ( log 2 x ) 1 / 3 \implies\frac{\log_2x}{3}=(\log_2x)^{1/3}

( log 2 x ) 3 27 = log 2 x \implies\frac{(\log_2x)^3}{27}=\log_2x

( log 2 x ) 2 = 27 . \implies(\log_2x)^2=\boxed{27}.

I understand your method right down to the second last line . From there you are multiplying the RHS by 27 and, after that, diving both sides by Log x to base 2 (?)

John Conway - 5 years, 3 months ago

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Cross multiplication. ( log 2 x ) 3 log 2 x = 27 \dfrac{(\log_2x)^3}{\log_2x} = 27

Hung Woei Neoh - 5 years, 2 months ago
Kartik Sharma
Nov 18, 2014

l o g 2 ( 1 3 l o g 2 x ) = 1 3 l o g 2 ( l o g 2 x ) {log}_{2}(\frac{1}{3} {log}_{2}x) = \frac{1}{3}{log}{2}({log}_{2}x)

3 l o g 2 1 3 + 3 l o g 2 ( l o g 2 x ) = l o g 2 ( l o g 2 x ) 3{log}_{2}\frac{1}{3} + 3{log}_{2}({log}_{2}x) = {log}_{2}({log}_{2}x)

2 l o g 2 ( l o g 2 x ) = l o g 2 27 2{log}_{2}({log}_{2}x) = {log}_{2} 27

l o g 2 x 2 = 2 l o g 2 27 {{log}_{2}x}^{2} = {2}^{{log}_{2} 27}

l o g 2 x 2 = 27 {{log}_{2}x}^{2} = 27

Vin Benzin
Jul 26, 2019

Matthew Dickinson
Nov 22, 2014

I can see why some find this somewhat tricky at first.

log((1/3) log(x)) = 1/3 log(log(x))

(1/3)*log(x) = (log(x))^(1/3)

(log(x))^3 = 27 (log(x))

(log(x))^2 = 27

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