Peculiar Logarithms

Level 1

log 3 5 × log 5 7 × log 7 9 × . . . × log 23 25 × log 25 27 = ? \log_{3}5 \times \log_{5}7 \times \log_{7}9 \times ... \times \log_{23}25 \times \log_{25}27 = ?


The answer is 3.000.

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1 solution

Chew-Seong Cheong
Aug 23, 2018

P = log 3 5 × log 5 7 × log 7 9 × × log 23 25 × log 25 27 Convert to same base = log 5 log 3 × log 7 log 5 × log 9 log 7 × × log 25 log 23 × log 27 log 25 = log 5 log 3 × log 7 log 5 × log 9 log 7 × × log 25 log 23 × log 27 log 25 = log 27 log 3 = 3 log 3 log 3 = 3 \begin{aligned} P & = \log_3 5 \times \log_5 7 \times \log_7 9 \times \cdots \times \log_{23} 25 \times \log_{25} 27 & \small \color{#3D99F6} \text{Convert to same base} \\ & = \frac {\log 5}{\log 3} \times \frac {\log 7}{\log 5} \times \frac {\log 9}{\log 7} \times \cdots \times \frac {\log 25}{\log 23} \times \frac {\log 27}{\log 25} \\ & = \frac {\cancel{\log 5}}{\log 3} \times \frac {\cancel{\log 7}}{\cancel{\log 5}} \times \frac {\cancel{\log 9}}{\cancel{\log 7}} \times \cdots \times \frac {\cancel{\log 25}}{\cancel{\log 23}} \times \frac {\log 27}{\cancel{\log 25}} \\ & = \frac {\log 27}{\log 3} = \frac {3\log 3}{\log 3} = \boxed 3 \end{aligned}

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