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Math Help - check my work

  1. #1
    Newbie
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    Dec 2006
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    check my work

    I am to evaluate the following:

    C(7,2)
    answer:21

    C(12,7)
    answer: 66

    This is the one I can't figure out

    the coefficient of x7y2 in the expansion of (2x-y)9

    any help is appreciated
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  2. #2
    Super Member

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    Lexington, MA (USA)
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    Hello, papa_chango123!

    Evaluate: . C(7,2)
    . . Answer: 21 . . . Right!

    C(12,7)
    . . Answer: 66 . . . no

    C(12,7) \:=\:\frac{12!}{7!5!} \:=\:\frac{12\cdot11\cdot10\cdot9\cdot8}{5\cdot4\c  dot3\cdot2\cdot1} \:=\:792



    Find the coefficient of x^7y^2 in the expansion of (2x-y)^9

    You're expected to know the Binomial Expansion.


    The term with x^7y^2 is: . C(9,7)(2x)^7(-y)^2

    Hence, we have: . \frac{9!}{7!2!}(2^7x^7)(y^2)\;=\;\boxed{4608}x^7y^  2

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