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Math Help - Combinatorics

  1. #1
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    Combinatorics

    Hi,

    I need a little bit help with this:

    Got to work my way from "sum from k=1 to n (k^2*n nCr k))" to this: (n^2+n)*2^(n-2), but HOW?

    Ive tried god-knows-what but nothing seems to work..
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  2. #2
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    Re: Combinatorics

    Quote Originally Posted by TempleT View Post
    Hi,

    I need a little bit help with this:

    Got to work my way from "sum from k=1 to n (k^2*n nCr k))" to this: (n^2+n)*2^(n-2), but HOW?

    Ive tried god-knows-what but nothing seems to work..
    Did you try calculating it for a couple of different values of n? When n= 1, the sum is 1^2(1)(1)(1)= 1. The right side you want to show that equal to is (1^2+ 2)(2^{-1})= \frac{3}{2}.

    You can't prove that, it isn't true.


    (Perhaps I am confused as to what "nCr" means. I thought that was the binomial coefficient but there is no "r" on the right, and it isn't an index so I assumed you meant "nCk".)
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  3. #3
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    Re: Combinatorics

    It it true, that i know for sure. (try wolfram alpha) I do mean the binomial coefficient. Im from Finland, maybe that is why you are confused with my nCr (we use that)
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  4. #4
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