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Math Help - binomial theorem factorial

  1. #1
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    binomial theorem factorial

    the first 3 terms in the expansion of (1 + ax) to the power n are 1+4x+7x to the power 2 ,find n and a
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  2. #2
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    Quote Originally Posted by joey1 View Post
    the first 3 terms in the expansion of (1 + ax) to the power n are 1+4x+7x to the power 2 ,find n and a

     <br />
(1+ax)^n=1+nax+\frac{n(n-1)}{2!}a^2x^2+...<br />

    nax=4x  , na=4 ---1

    \frac{n(n-1)}{2!}a^2=7 ---2

    Solve the simultaneous equation then .
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