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Math Help - Can't do combinatorial proofs. Can someone help?

  1. #1
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    Can't do combinatorial proofs. Can someone help?

    Sorry I don't know how to type this in mathematical symbols.
    Prove that:

    the summation from r=0 to n of (C(n,r))(a-1)^r = a^n

    Thanks for any help I can get
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  2. #2
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    Use the binomial theorem.
    \left( {x + y} \right)^n  = \sum\limits_{r = 0}^n {\binom{n}{r}x^r y^{n - r} }

    Let x=(a-1)~\&~y=1
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