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Is there an expansion for the following summation? $ (a_1 + a_2 + \cdots + a_k)^n $

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http://en.wikipedia.org/wiki/Multinomial_theorem

This is what you seek.

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    @muad: Not necessarily. This link perfectly answers the question at hand. See [this](http://meta.math.stackexchange.com/questions/24/answers-that-simply-link-to-a-paper-with-little-or-no-content-in-the-answer-itse) meta thread.2010-10-20
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Since you know what $(a+b)^{n}$ is you can take $A = a_{1} + a_{2} + \cdots + a_{k-1}$ and $B=a_{k}$ and try to simplify the big expression by the binomial theorem.

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    @M.S: Yes, i agree!2010-10-20