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Possible Duplicate:
How to find the sum of this infinite series

How can I calculate result of $\sum\limits_{i=1}^n\cfrac{n}{2^n}$ ?

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    $\begingroup$ similar question math.stackexchange.com/questions/50919/… $\endgroup$ – Andrew Oct 4 '11 at 17:51
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    $\begingroup$ In the same vein as the two questions cited above:$$\sum_{n=1}^\infty n x^n=\frac{x}{(1-x)^2}$$ $\endgroup$ – robjohn Oct 4 '11 at 18:06
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    $\begingroup$ In above links the upper limit is infinity.But here is n. $\endgroup$ – user16948 Oct 4 '11 at 18:19
  • $\begingroup$ And the index of summation is $i$. This is somewhat confusing... $\endgroup$ – user940 Oct 4 '11 at 19:03