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Math Help - System of linear equations.

  1. #1
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    System of linear equations.

    how do i solve for this system of equations

    11x-3y=-36
    12y-9x=39
    ?
    tried reordering the terms but what else?
    Last edited by mr fantastic; August 10th 2010 at 04:03 AM. Reason: Re-titled.
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  2. #2
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    Quote Originally Posted by gabriel View Post
    how do i solve for this system of equations

    11x-3y=-36
    12y-9x=39
    ?
    tried reordering the terms but what else?
    11x-3y=-36 ...(1)
    12y-9x=39 ...(2)

    From (1) 11x-3y=-36\implies -3y=-36-11x \implies y=12+\frac{11}{3}x

    Then (2) 12y-9x=39\implies 12\left(12+\frac{11}{3}x\right)-9x=39 Can you finish it from here?
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  3. #3
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    yeppp i got thnx good refreshment to the dusty mind
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  4. #4
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    Quote Originally Posted by gabriel View Post
    how do i solve for this system of equations

    11x-3y=-36
    12y-9x=39
    ?
    tried reordering the terms but what else?
    The alternative method (the method used for more equations with more unknowns) is the elimination method...

    11x - 3y = -36
    -9x + 12y = 39.

    Multiply the first equation by 4 to give

    44x-12y=-144
    -9x+12y=39

    Add the equations together

    (44x-12y)+(-9x+12y)=-144+39

    35x=-105

    x = -3.

    Substitute into one of the original equations...

    11(- 3) - 3y = -36

    -33- 3y = -36

    -3y = -3

    y = 1


    So the solution is (x,y) = (-3,1).
    Last edited by Prove It; August 9th 2010 at 11:02 PM.
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  5. #5
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    Quote Originally Posted by Prove It View Post
    ...

    (44x-12y)+(-9x+12y)=-144+39

    35x=105 <=== unfortunately you made a small sign mistake here

    x = 3.

    Substitute into one of the original equations...

    11\cdot 3 - 3y = -36

    33- 3y = -36

    -3y = -69

    y = 23.


    So the solution is (x,y) = (3,23).
    (44x-12y)+(-9x+12y)=-144+39

    35x=-105

    x = -3.
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  6. #6
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    Yes I did, edited
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