My Cyclic Logarithmic Sum

Algebra Level 1

cyclic ( a , b , c ) ( 1 1 + log a b c ) = ? \large \sum_{\text{cyclic}(a,b,c)} \left( \dfrac1{1 + \log_a bc} \right) = \, ?


The answer is 1.

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5 solutions

Hung Woei Neoh
Apr 21, 2016

cyclic ( a , b , c ) ( 1 1 + log a b c ) \displaystyle \sum_{\text{cyclic}(a,b,c)} \left( \dfrac{1}{1+\log_a bc} \right)

= 1 1 + log a b c + 1 1 + log b c a + 1 1 + log c a b =\dfrac{1}{1+\log_a bc} + \dfrac{1}{1+\log_b ca} + \dfrac{1}{1+\log_c ab}

= 1 1 + log a b c + 1 1 + log a c a log a b + 1 1 + log a a b log a c =\dfrac{1}{1+\log_a bc} + \dfrac{1}{1+\dfrac{\log_a ca}{\log_a b}} + \dfrac{1}{1+\dfrac{\log_a ab}{\log_a c}}

= 1 1 + log a b c + 1 ( log a b + log a c + log a a log a b ) + 1 ( log a c + log a a + log a b log a c ) =\dfrac{1}{1+\log_a bc} + \dfrac{1}{\left(\dfrac{\log_a b + \log_a c + \log_a a}{\log_a b}\right)} + \dfrac{1}{\left(\dfrac{\log_a c + \log_a a + \log_a b}{\log_a c}\right)}

= 1 1 + log a b c + log a b 1 + log a b c + log a c 1 + log a b c =\dfrac{1}{1+\log_a bc} + \dfrac{\log_a b}{1 + \log_a bc} + \dfrac{\log_a c}{1 + \log_a bc}

= 1 + log a b c 1 + log a b c =\dfrac{1 + \log_a bc}{1 + \log_a bc}

= 1 =\boxed{1}

Rishabh Jain
Apr 5, 2016

Using log a b = ln b ln a \color{#D61F06}{\log_a b=\dfrac{\ln b}{\ln a}} and ln ( a b ) = ln a + ln b \color{#3D99F6}{\ln (ab)=\ln a+\ln b} summation can be written as:

cyc 1 1 + ln b c ln a = cyc ln a ln a + ln b c = cyc ln a ln a + ln b + ln c = cyc ln a ln a + ln b + ln c = 1 \large{\begin{aligned}&\displaystyle\sum_{\text{cyc}}\dfrac{1}{1+\dfrac{\ln bc}{\ln a}}\\&=\displaystyle\sum_{\text{cyc}}\dfrac{\ln a}{\ln a+\ln bc}\\&=\displaystyle\sum_{\text{cyc}} \dfrac{\ln a}{\ln a+\ln b+\ln c}\\&=\dfrac{\displaystyle\sum_{\text{cyc}}\ln a}{\ln a+\ln b+\ln c}\\&\huge = \boxed 1 \end{aligned}}

Yeah C O O L \mathfrak{COOL}

Aditya Narayan Sharma - 5 years, 2 months ago

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T H A N K S \large\mathbf{THANKS}

Rishabh Jain - 5 years, 2 months ago

Nice thinking good job.

Vikash Kumar - 4 years, 10 months ago

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Thanks... :-D

Rishabh Jain - 4 years, 10 months ago

how do you color words?

Oximas omar - 1 month ago
Ruben Wiechers
Apr 15, 2017

cyclic ( a , b , c ) 1 1 + log a ( b c ) = cyclic ( a , b , c ) 1 log a ( a ) + log a ( b c ) = cyclic ( a , b , c ) 1 log a ( a b c ) = cyclic ( a , b , c ) log a b c ( a ) = log a b c ( a ) + log a b c ( b ) + log a b c ( c ) = log a b c ( a b c ) = 1 \displaystyle \sum_{\text{cyclic}(a,b,c)} \frac{1}{1+\log_a (bc)} \\ =\displaystyle \sum_{\text{cyclic}(a,b,c)} \frac{1}{\log_a (a)+\log_a (bc)} \\ \displaystyle = \sum_{\text{cyclic}(a,b,c)} \frac{1}{\log_a (abc)} \\ \displaystyle = \sum_{\text{cyclic}(a,b,c)} \log_{abc} (a) \\ = \log_{abc} (a) + \log_{abc} (b) + \log_{abc} (c) \\ = \log_{abc} (abc) \\ = \boxed{1}

Kano Boom
Dec 5, 2019

The equation above is true for any positive values of a,b,c. (guess) So just sub in a=b=c=2 and you get the answer

Akshay Krishna
Dec 25, 2018

If it's about solving a problem--then be it a = b = c a=b=c for this case (take 2 for example-or anything valid for this function).

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