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( 2 ) log 2 2 log 2 2 log 2 2 log 2 2 log 2 x = 5 \Large \Big(\sqrt{2}\Big)^{\log_{2}2^{\log_{2}2^{\log_{2}2^{\log_{2}2^{\log_{2}x}}}}}=\ 5

What is the value of x ? x?

5 16 25 32

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3 solutions

Chew-Seong Cheong
Jun 21, 2018

( 2 ) log 2 2 log 2 2 log 2 2 log 2 2 log 2 x = 5 Note that log 2 2 log 2 x = log 2 x log 2 2 = log 2 x ( 2 ) log 2 2 log 2 2 log 2 2 log 2 x = 5 ( 2 ) log 2 2 log 2 2 log 2 x = 5 ( 2 ) log 2 2 log 2 x = 5 ( 2 ) log 2 x = 5 2 1 2 log 2 x = 5 Taking log 2 on both sides 1 2 log 2 x = log 2 5 log 2 x = 2 log 2 5 = log 2 25 x = 25 \large \begin{aligned} (\sqrt 2)^{\log_2 2^{\log_2 2^{\log_2 2^{\log_2 2^{\log_2 x}}}}} & = 5 & \small \color{#3D99F6} \text{Note that }\log_2 2^{\log_2 x} = \log_2 x \log_2 2 = \log_2 x \\ (\sqrt 2)^{\log_2 2^{\log_2 2^{\log_2 2^{\log_2 x}}}} & = 5 \\ (\sqrt 2)^{\log_2 2^{\log_2 2^{\log_2 x}}} & = 5 \\ (\sqrt 2)^{\log_2 2^{\log_2 x}} & = 5 \\ (\sqrt 2)^{\log_2 x} & = 5 \\ 2^{\frac 12 \log_2 x} & = 5 & \small \color{#3D99F6} \text{Taking }\log_2 \text{ on both sides} \\ \frac 12 \log_2 x & = \log_2 5 \\ \log_2 x & = 2\log_2 5 \\ & = \log_2 25 \\ \implies x & = \boxed{25} \end{aligned}

Continuously using the this particular property of logarithms -> log a b c = c l o g a b \log_{a}b^{c} = c\cdot log_{a}b and l o g a a = 1 log_{a}a =1 ,

The log 2 x \log_{2}x keep moving down until the equation becomes -> 2 l o g 2 x \sqrt{2}^{log_{2}x} = 5

Squaring both sides => 2 l o g 2 x = 25 2^{log_{2}x} = 25

l o g 2 x {log_{2}x} basically asks 2 to the power what equals ‘x’.

So, 2 l o g 2 x = x 2^{log_{2}x} = x .

Therefore the equation is true when x = 25 \textbf{x = 25}

Ritabrata Roy
Jun 24, 2018

That's simple ,just use a^log(c) base a=C from the top corner

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