Fun with logarithms!

Algebra Level 3

Solve for x x : log 2 ( 2 x 1 ) + x = log 4 144 \large \log_{2}{(2^{x}-1)}+x=\log_{4}{144}

0 2 3 None of these. 4

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2 solutions

Rohit Udaiwal
Nov 13, 2015

We have log 2 ( 2 x 1 ) + x = log 4 144 log 2 ( 2 x 1 ) + x = log 2 2 1 2 2 = log 2 12 x = log 2 12 log 2 ( 2 x 1 ) x = log 2 ( 12 2 x 1 ) 2 x = 12 2 x 1 \log_{2}{(2^{x}-1)}+x=\log_{4}{144} \\ \implies \log_{2}{(2^x-1)}+x=\log_{2^2}{12^2}=\log_{2}{12}\\ \implies x=\log_{2}{12 }-\log_{2}{(2^x-1)} \\ \implies x=\log_{2}{\left (\dfrac {12}{2^x-1}\right)} \\\implies 2^x=\dfrac {12}{2^x-1} Let 2 x 2^x be a a .The equation becomes : a = 12 a 1 a 2 a 12 = 0 ( a 4 ) ( a + 3 ) = 0 a = 4 , 3 but 2 x = 3 is undefined. 2 x = 4 x = 2 a=\dfrac {12}{a-1} \implies a^2-a-12=0 \implies (a-4)(a+3)=0 \implies a=4,-3 \\\text{but}~2^x=-3~ \text{is undefined.}\\ \implies 2^x=4 \therefore \boxed {x=2}

Chew-Seong Cheong
Sep 11, 2018

log 2 ( 2 x 1 ) + x = log 4 144 = log 2 144 log 2 4 = log 2 2 4 3 2 log 2 2 2 = 4 + 2 log 2 3 2 log 2 ( 2 x 1 ) + x = 2 + log 2 3 \begin{aligned} \log_2(2^x-1) + x & = \log_4 144 \\ & = \frac {\log_2 144}{\log_2 4} \\ & = \frac {\log_2 2^4 \cdot 3^2}{\log_2 2^2} \\ & = \frac {4 + 2\log_2 3}2 \\ \implies \log_2 ({\color{#3D99F6}2^x-1}) + {\color{#D61F06}x} & = {\color{#D61F06}2} + \log_2 \color{#3D99F6}3 \end{aligned}

Equating respective terms on both sides { 2 x 1 = 3 2 x = 4 x = 2 x = 2 \implies \begin{cases} \color{#3D99F6} 2^x - 1 = 3 & \implies 2^x = 4 & \implies x = \boxed 2 \\ \color{#D61F06} x = \boxed 2 \end{cases}

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