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"Prove that" question concerning combinations Watch

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    Hello everyone!

    Could anyone please let me know how do I do this question? I have no clue how to start here (I have some brief workings but they didn't prove to be of any use). I know the general term but can't figure out how to use the given identity to prove the given equation.



    Thank you very much!

    Sid
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    The question basically wants you to find the binomial coefficient in front of x^r, in the right hand side of the equation in terms of the binomial coefficients of the left side of the equations.

    So what terms in the left hand side of the equation make up x^r in the right hand side of the equation?
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    Ah! Got it!

    Thanks!
 
 
 
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