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Math Help - Solving simultaneous equations.

  1. #1
    m58
    m58 is offline
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    Solving simultaneous equations.

    Hey guys,

    This problem has been driving me crazy: If 2x - y = 9 and 3x + 4y = -14, then x = ? and y = ?

    I keep getting y = -9 and x = 7.33 by doing this:

    2x - y = 9 -> 2x - 9 = y -> 2(0) - 9 = y -> -9 = y

    then

    3x + 4(-9) = -14 -> 3x + -36 = -14 -> 3x = 22 -> x = 7.33

    I know these are wrong, though. Would anyone be able to tell me what's wrong?

    Thanks in advance!
    Last edited by mr fantastic; September 12th 2011 at 01:15 AM. Reason: Re-titled.
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  2. #2
    Member anonimnystefy's Avatar
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    Re: Basic algebra problem

    hi

    you cannot assume that x=0,because if it isn't it is going to give you wrong answers.

    you can try doing something like this:

    2x-y=9
    -y=9-2x
    y=-9+2x
    y=2x-9

    we substitute into the second equation:

    3x+4(2x-9)=-14
    3x+8x-36=--14
    11x=36-14
    11x=22
    x=2

    can you find y now?
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