Can we cut off everything as well?

Algebra Level 2

r = 10 99 log r ( r + 1 ) = ? \large \prod_{r=10}^{99} \log_r (r+1) = \, ?


The answer is 2.

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1 solution

Chew-Seong Cheong
Jan 22, 2016

r = 10 99 log r ( r + 1 ) = log 10 ( 11 ) ˙ log 11 ( 12 ) ˙ log 12 ( 13 ) ˙ . . . ˙ log 99 ( 100 ) = log 10 ( 11 ) log 10 ( 10 ) ˙ log 10 ( 12 ) log 10 ( 11 ) ˙ log 10 ( 13 ) log 10 ( 12 ) ˙ . . . ˙ log 10 ( 100 ) log 10 ( 99 ) = log 10 ( 100 ) log 10 ( 10 ) = 2 \begin{aligned} \prod_{r=10}^{99} \log_r (r+1) & = \log_{10} (11) \dot{} \log_{11} (12) \dot{} \log_{12} (13) \dot{} ... \dot{} \log_{99} (100) \\ & = \frac{\log_{10} (11)}{\log_{10} (10)} \dot{} \frac{\log_{10} (12)}{\log_{10} (11)} \dot{} \frac{\log_{10} (13)}{\log_{10} (12)} \dot{} ... \dot{} \frac{\log_{10} (100)}{\log_{10} (99)} \\ & = \frac{\log_{10} (100)}{\log_{10} (10)} \\ & = \boxed{2} \end{aligned}

best solution

aishwarya dadhich - 5 years, 4 months ago

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