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Math Help - Solve via elimination

  1. #1
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    Solve via elimination

    I think I am doing this problem wrong. To solve by elimination:
    0.3x-0.2y=4 and 0.4x +0.3y=-1. After changing to integers, I multiplied the 1st equation by 4 and the second by 3 ending up with 8x-11y=17. I do not know where to go from here, or if I did this correctly...
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  2. #2
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    You have done the first step successfully: multiplying the first equation by 4 gives you 1.2x - 0.8y =16, and multiplying the second equation by 3 gives you 1.2x + 0.9y = -3.

    Now, apply the elimination method. I'm not sure where you got the result you have. If you are having trouble understanding the elimination method, try Googling for "solve equations by elimination"
    Last edited by borophyll; May 26th 2011 at 11:49 PM.
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  3. #3
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    Thank you. Just to be sure: I now add both equtions. Then, how do I solve for Y? Do I divide each side? This is where I'm unsure...
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  4. #4
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    Quote Originally Posted by jay1 View Post
    I think I am doing this problem wrong. To solve by elimination:
    0.3x-0.2y=4 and 0.4x +0.3y=-1. After changing to integers, I multiplied the 1st equation by 4 and the second by 3 ending up with 8x-11y=17. I do not know where to go from here, or if I did this correctly...
    You should review how you ended up with 8x-11y=17.

    To solve by elimination, first decide whether or not you want to
    eliminate x or y.
    Having eliminated one, you will find the value of the other.

    3x-2y=40 after multiplying both sides by 10.
    4x+3y=-10

    If we choose to eliminate y, we can obtain -6y on the first line
    and +6y on the second line.

    How do arrive at that ?
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  5. #5
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    Quote Originally Posted by Archie Meade View Post
    You should review how you ended up with 8x-11y=17.

    To solve by elimination, first decide whether or not you want to
    eliminate x or y.
    Having eliminated one, you will find the value of the other.

    3x-2y=40 after multiplying both sides by 10.
    4x+3y=-10

    If we choose to eliminate y, we can obtain -6y on the first line
    and +6y on the second line.

    How do arrive at that ?
    By multiplying the top equation by -2 and the bottom by 3. I then end up with -1x = 100. Is this right?
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  6. #6
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    Quote Originally Posted by jay1 View Post
    By multiplying the top equation by -2 and the bottom by 3. I then end up with -1x = 100. Is this right?
    No, that doesn't work.

    Multiplying both sides of the equation 3x-2y=40 by 3 gives 9x-6y=120.
    Multiplying both sides of 4x+3y=-10 by 2 gives 8x+6y=-20.

    Now, if you add (9x-6y) to (8x+6y) you eliminate y.
    The number of x you get will be 120-20, but how many x is that ?
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