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Math Help - Solving Systems of Equations by Elimination Help

  1. #1
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    Solving Systems of Equations by Elimination Help

    Hey, I understand the concept for this, but for some reason I can't seem to get the answers right. I don't know what I'm doing wrong. This is one of the problems I did and my work.

    4x + 8y = 20
    -4x + 2y = -30

    4x + 8y/8 = 20/8
    4x/4x + y = 2.5/4x
    y = 2.5/4x

    4x + 8(2.5/4x) = 20
    4x/4 + 4.75 = 20/4
    x + 4.75 -4.75 = 5 - 4.75
    x = 0.25

    4(.25) + 8y = 20
    1 + 8y/8 = 20/8
    1 -1 + y = 2.2 - 1
    y = 1.2

    CHECK:

    4x + 8y = 20
    4(.25) + 8(1.2) = 20
    1 + 9.6 =/= 20
    10.6 =/= 20

    Thanks in advance!
    Last edited by drakojan201; October 6th 2012 at 05:17 PM.
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  2. #2
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    Re: Solving Systems of Equations by Elimination Help

    First of all, it appears you are trying to use the substitution method, which is the long way in this case considering the x values can be eliminated immediately by adding the two equations.

    Anyway, your mistakes come from when you are dividing. In an equation, when you need to undo a multiplication using division, to do ANY division, it is ALWAYS done to EVERYTHING on both sides, not just to one term.
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    Re: Solving Systems of Equations by Elimination Help

    Really? I did another problem where I only divided one on each side and the answer was right.
    4x/4x + y = -3/4
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    Re: Solving Systems of Equations by Elimination Help

    elimination method: add the terms ...

    4x + 8y = 20
    -4x + 2y = -30
    ---------------
    ... 10y = -10

    y = -1

    now use the solution for y either of the original equations to find x
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  5. #5
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    Re: Solving Systems of Equations by Elimination Help

    Then please tell us what that problem was!
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    Re: Solving Systems of Equations by Elimination Help

    -4x - 2y = -12
    4x + 8y/8 = -24/8
    4x/4x + y = -3/4x
    y = -3/4x

    4x + 8(-3/4x) = -24
    4x - 6 + b = -24 + b
    4x/4 = 18/4

    x = 4.5 (I noticed that I made a mistake or two, but when I checked it, everything was right so I just left it.)

    4(4.5) + 8y = -24
    18 -18 + 8y = -24 - 18
    8y/8 = 42/8
    y = 6

    4(4.5) + 8(6) = -24
    18 - 42 = -24
    -24 = -24

    -4(4.5) - 2(-6) = -12
    -18 + 6 = -12
    -12 = -12

    x = 4.5
    y = 6
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    Re: Solving Systems of Equations by Elimination Help

    Quote Originally Posted by drakojan201 View Post
    -4x - 2y = -12
    4x + 8y/8 = -24/8
    4x/4x + y = -3/4x
    y = -3/4x
    You seem unaware of the basics...better start listeming in math class

    Start by getting the y-term by itself:
    2y = -4x + 12
    Divide by 2:
    y = -2x + 6
    That's it.
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