Touching Logarithm After Long Days

Algebra Level 3

1 1 + log a 2 b c a + 1 1 + log c 2 a b c + 1 1 + log b 2 c a b \large{\dfrac{1}{1+\log_{a^2b}{\dfrac{c}{a}}}+\dfrac{1}{1+\log_{c^2a}{\dfrac{b}{c}}}+\dfrac{1}{1+\log_{b^2c}{\dfrac{a}{b}}}}

If a , b , c R > 1 a,b,c \in \mathbb{R} >1 , find the value of the expression above.


The answer is 3.

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2 solutions

Ruben Wiechers
Apr 20, 2017

I like to swap bases instead of using a new base : log a b = 1 log b a \log_a{b}=\frac 1{\log_b{a}}

S = 1 1 + log a 2 b ( c a ) + 1 1 + log c 2 a ( b c ) + 1 1 + log b 2 c ( a b ) = 1 log a 2 b ( a 2 b ) + log a 2 b ( c a ) + 1 log c 2 a ( c 2 a ) + log c 2 a ( b c ) + 1 log b 2 c ( b 2 c ) + log b 2 c ( a b ) = 1 log a 2 b ( a b c ) + 1 log c 2 a ( a b c ) + 1 log b 2 c ( a b c ) = log a b c ( a 2 b ) + l o g a b c ( b 2 c ) + log a b c ( c 2 a ) = log a b c ( a b c ) 3 = 3 \begin{aligned} S & = \frac 1{1+\log_{a^2b} (\frac ca)} + \frac 1{1+\log_{c^2a} (\frac bc)} + \frac 1{1+\log_{b^2c} (\frac ab)} \\ &= \frac 1{\log_{a^2b}(a^2b) + \log_{a^2b} (\frac ca)} + \frac 1{\log_{c^2a}(c^2a)+\log_{c^2a} (\frac bc)} + \frac 1{\log_{b^2c}(b^2c)+\log_{b^2c} (\frac ab)} \\ &= \frac 1{\log_{a^2b}(abc)} + \frac 1{\log_{c^2a}(abc)} + \frac 1{\log_{b^2c}(abc)} \\ &= \log_{abc}(a^2b) + log_{abc}(b^2c)+\log_{abc}(c^2a) \\ &= \log_{abc}(abc)^3 \\ &= \boxed{3} \end{aligned}

Chew-Seong Cheong
Apr 19, 2017

S = 1 1 + log a 2 b c a + 1 1 + log c 2 a b c + 1 1 + log b 2 c a b = 1 1 + log c log a 2 log a + log b + 1 1 + log b log c 2 log c + log a + 1 1 + log a log b 2 log b + log c = 2 log a + log b 2 log a + log b + log c log a + 2 log c + log a 2 log c + log a + log b log c + 2 log b + log c 2 log b + log c + log a log b = 2 log a + log b log a + log b + log c + 2 log c + log a log a + log b + log c + 2 log b + log c log a + log b + log c = 3 log a + 3 log b + 3 log c log a + log b + log c = 3 \begin{aligned} S & = \frac 1{1+\log_{a^2b} \frac ca} + \frac 1{1+\log_{c^2a} \frac bc} + \frac 1{1+\log_{b^2c} \frac ab} \\ & = \frac 1{1+\frac {\log c - \log a}{2\log a + \log b}} + \frac 1{1+\frac {\log b - \log c}{2\log c + \log a}} + \frac 1{1+\frac {\log a - \log b}{2\log b + \log c}} \\ & = \frac {2\log a + \log b}{2\log a + \log b+ \log c - \log a} + \frac {2\log c + \log a}{2\log c + \log a+ \log b - \log c} + \frac {2\log b + \log c}{2\log b + \log c + \log a - \log b} \\ & = \frac {2\log a + \log b}{\log a + \log b+ \log c} + \frac {2\log c + \log a}{\log a + \log b+ \log c} + \frac {2\log b + \log c}{\log a + \log b+ \log c} \\ & = \frac {3\log a + 3\log b+ 3\log c}{\log a + \log b+ \log c} \\ & = \boxed{3} \end{aligned}

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