Logarithms

Algebra Level 3

Find the value of 6 log 25 360 + 24 log 3 360 + 9 log 16 360 \dfrac{6}{\log_{25}{360}}\ + \dfrac{24}{\log_{3}{360}}\ + \dfrac{9}{\log_{16}{360}} .


The answer is 12.

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2 solutions

Zach Abueg
Jul 6, 2017

6 log 25 360 + 24 log 3 360 + 9 log 16 360 = 6 log 25 + 24 log 3 + 9 log 16 log 360 = 12 log 5 + 24 log 3 + 36 log 2 log 360 = 12 log 5 + 2 log 3 + 3 log 2 log 360 = 12 log 5 + log 9 + log 8 log ( 5 9 8 ) = 12 \displaystyle \begin{aligned} \frac{6}{\log_{25} 360} + \frac{24}{\log_3 360} + \frac{9}{\log_{16} 360} & = \frac{6\log 25 + 24\log 3 + 9\log 16}{\log 360} \\ & = \frac{12\log 5 + 24\log 3 + 36\log 2}{\log 360} \\ & = 12 \ \frac{\log 5 + 2\log 3 + 3\log 2}{\log 360} \\ & = 12 \ \frac{\log 5 + \log 9 + \log 8}{\log (5 \cdot 9 \cdot 8)} \\ & = \boxed{12} \end{aligned}

Excellent approach

azadali jivani - 3 years, 10 months ago
Daniel Lee
Jul 6, 2017

We use the fact that 1 log a b = log b a \frac{1}{\log_{a}{b}} = \log_{b}{a} . So 6 log 25 360 + 24 log 3 360 + 9 log 16 360 = log 360 2 5 6 + log 360 3 24 + log 360 1 6 9 = log 2 3 3 2 5 ( 5 12 3 24 2 36 ) = 12 \frac{6}{\log_{25}{360}}\ + \frac{24}{\log_{3}{360}}\ + \frac{9}{\log_{16}{360}} = \log_{360}{25^{6}} + \log_{360}{3^{24}} + \log_{360}{16^{9}} = \log_{2^{3}3^{2}5}{(5^{12}3^{24}2^{36})} = \boxed{12} .

nice solutions

azadali jivani - 3 years, 10 months ago

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