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Thread: Counting-Combinations

  1. March 26th 2011, 08:06 AM #1
    qwerty10
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    Counting-Combinations

    I am having problems understanding as to why n+r-1 C n-1= n+r-1 C r?
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  2. March 26th 2011, 08:16 AM #2
    Plato
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    Quote Originally Posted by qwerty10 View Post
    I am having problems understanding as to why n+r-1 C n-1= n+r-1 C r?
    Because \dbinom{N}{k}=\dbinom{N}{N-k}.

    (n+r-1)-(n-1)=r
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