Saturday, 19 December 2015

Infinite Series sum1/(n(n+1))



I am confused on the following series:



n=11n(n+1)=1




My calculator reveals that the answer found when evaluating this series is 1. However, I am not sure how it arrives at this conclusion. I understand that partial fractions will be used creating the following equation. I just don't understand how to proceed with the problem.



n=1(1n1n+1)=1


Answer



Write out a few terms of the series. You should see a pattern! But first consider the finite series:



mn=1(1n1n+1)=112+1213+1314++1m11m+1m1m+1.


This sum is telescoping, since it collapses like a telescope.




Everything is left except for the first and last term. Now what's the limit as m?


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