Problem 3

Algebra Level 3

r = 2 63 log 2 ( 1 + 1 r ) = ? \sum_{r=2}^{63} \log_2 \left( 1 + \dfrac1r \right) =\, ?


The answer is 5.

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2 solutions

r = 2 63 log 2 ( 1 + 1 r ) = log 2 ( r = 2 63 ( 1 + 1 r ) ) = log 2 ( 3 2 4 3 5 4 64 63 ) = \sum_{r=2}^{63} \log_2 \left( 1 + \dfrac1r \right) = \log_2(\prod_{r=2}^{63} (1 + \dfrac1r)) = \log_2(\dfrac 32 \cdot \dfrac 43 \cdot \dfrac 54 \cdot \ldots \cdot \dfrac {64}{63}) = log 2 64 2 = log 2 32 = 5 \log_2 \frac {64}{2} = \log_2 32 = 5

Kay Xspre
Jan 25, 2016

l o g 2 ( 1 + 1 r ) = l o g 2 ( r + 1 r ) = l o g 2 ( r + 1 ) l o g 2 ( r ) log_2(1+\frac{1}{r}) = log_2(\frac{r+1}{r}) = log_2(r+1)-log_2(r) From here we just put sigma into it and the solution will be ( l o g 2 64 l o g 2 63 ) + ( l o g 2 63 l o g 2 62 ) + + ( l o g 2 3 l o g 2 2 ) = l o g 2 64 l o g 2 2 = 6 1 = 5 (log_2 64-log_2 63)+(log_2 63-log_2 62)+\dots+(log_2 3-log_2 2) = log_2 64 - log_2 2 = 6-1 = 5

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