Nested Logarithms

Algebra Level 3

log 2 x + log 2 x + log 2 x + + log 2 x 7 terms = 1016 \underbrace{\log_{\sqrt2}{x} + \log_{\sqrt{\sqrt2}}{x} + \log_{\sqrt{\sqrt{\sqrt2}}}{x} +\cdots + \log_{\sqrt{\sqrt{\sqrt{\sqrt{\sqrt{\sqrt{\sqrt 2}}}}}}}x}_{\text{7 terms}} = 1016

Find the value of x x .


The answer is 16.

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2 solutions

Anuj Shikarkhane
Jun 17, 2018

The given expression simplifies to

log 2 1 2 x + log 2 1 4 x + + log 2 1 128 x = 1016 \log_{2^{\frac{1}{2}}}{x} + \log_{2^{\frac{1}{4}}}{x} + \cdots + \log_{2^{\frac{1}{128}}}{x} = 1016 .

We know that log a m = log b m log b a \log_{a}{m}= \dfrac{\log_{b}{m}}{\log_{b}{a}} If a 1 a \neq 1 , b 1 b \neq 1 . We take the base b b as 2 2 .

log 2 x log 2 2 1 2 + log 2 x log 2 2 1 4 + + log 2 x log 2 2 1 128 = 1016 \implies \dfrac{\log_{2}{x}}{\log_{2}{2^{\frac{1}{2}}}} + \dfrac{\log_{2}{x}}{\log_{2}{2^{\frac{1}{4}}}} + \cdots + \dfrac{\log_{2}{x}}{\log_{2}{2^{\frac{1}{128}}}} = 1016

log 2 x 1 2 + log 2 x 1 4 + + log 2 x 1 128 = 1016 \implies \dfrac{\log_{2}{x}}{\dfrac{1}{2}} + \dfrac{\log_{2}{x}}{\dfrac{1}{4}} + \cdots + \dfrac{\log_{2}{x}}{\dfrac{1}{128}}=1016 .

2 × log 2 x + 4 × log 2 x + 8 × log 2 x + + 128 × log 2 x = 1016 \implies 2\times \log_{2}{x} + 4\times \log_{2}{x} + 8\times \log_{2}{x} + \cdots + 128 \times \log_{2}{x}=1016 .

log 2 x × ( 2 + 4 + 8 + 16 + 32 + 64 + 128 ) = 1016 \implies \log_{2}{x}\times \left(2+4+8+16+32+64+128\right) =1016 .

log 2 x × 254 = 1016 \implies \log_{2}{x} \times 254 = 1016 .

log 2 x = 4 \implies \log_{2}{x} = 4 .

x = 16 \implies x = 16

Just like other function such as \frac xy x y \frac xy or \sqrt 2 2 \sqrt 2 , you need to put a backslash \ before \log_{\sqrt x} log x \log_{\sqrt x} , \tan x tan x \tan x \sin x sin x \sin x \cos x cos x \cos x and all that. Note that function names should not be in italic which is for constants and variables. Also note that there is a space between the function name and variable log x l o g x log x (see no space even if you put in a few) but \log x log x \log x (there is a space).

Chew-Seong Cheong - 2 years, 11 months ago

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Thank you. I've edited it accordingly.

Anuj Shikarkhane - 2 years, 11 months ago
Chew-Seong Cheong
Jun 26, 2018

log 2 x + log 2 x + log 2 x + + log 2 x = 1016 log x 1 2 log 2 + log x 1 4 log 2 + log x 1 8 log 2 + + log x 1 2 7 log 2 = 1016 log x log 2 ( 2 + 2 2 + 2 3 + + 2 7 ) = 1016 log x log 2 × 2 × 2 7 1 2 1 = 1016 log x log 2 = 4 log x = 4 log 2 x = 2 4 = 16 \begin{aligned} \log_{\sqrt2}{x} + \log_{\sqrt{\sqrt2}}{x} + \log_{\sqrt{\sqrt{\sqrt2}}}{x} +\cdots + \log_{\sqrt{\sqrt{\sqrt{\sqrt{\sqrt{\sqrt{\sqrt 2}}}}}}}x & = 1016 \\ \frac {\log x}{\frac 12 \log 2} + \frac {\log x}{\frac 14 \log 2} + \frac {\log x}{\frac 18 \log 2} + \cdots + \frac {\log x}{\frac 1{2^7} \log 2} & = 1016 \\ \frac {\log x}{\log 2} (2+2^2+2^3+\cdots + 2^7) & = 1016 \\ \frac {\log x}{\log 2} \times 2 \times \frac {2^7-1}{2-1} & = 1016 \\ \frac {\log x}{\log 2} & = 4 \\ \log x & = 4 \log 2 \\ \implies x & = 2^4 = \boxed{16} \end{aligned}

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