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Math Help - Factorizing

  1. #1
    Junior Member
    Joined
    Jan 2009
    Posts
    28

    Factorizing

    I'm not sure how to do factorize this problem:
    X^2 + 4X - 21

    The answer is:
    (X-3)(X+7)

    So I tried to split it up like this:
    X(X+1)-3(X+7)

    .. but I'm not sure how to group it.

    Help please?
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  2. #2
    Senior Member
    Joined
    Nov 2007
    Posts
    329
    You should find its roots. 3^2+4\cdot 3-21=0\text{ and }(-7)^2-7\cdot 4-21=0
    Do you know the quadratic formula? If not you can still figure the factor out: first (x\pm?)(x\pm?) second: ?\cdot ? must be -21 so you try with integers and get it soon.
    Last edited by james_bond; January 29th 2009 at 09:11 AM.
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