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Solving System of Equations.

  1. Sep 14, 2003 #1
    I found this System of Equations to be a real pain, ive come up with alot various answers and some of them have worked, but then when i try it on others, they dont work.

    x + 2y = 5
    3x +4z = 2
    2y + 3w = -2
    3z - 2w = 1

    Ive tried solving this so many times. Ive gotten z = 29/35, x= 323/35, y = -74/35, w = 26/35. They work on some, but then it doesn't work on the x. Ive also gotten w = 17/35, x = -22/105, z = 23/35, and y = 547/210. Evidently i'm making some mistakes in some areas of solving this system. Any mistake can cause huge catastrophe in Math. Please help me out here.
     
  2. jcsd
  3. Sep 14, 2003 #2
    x + 2y = 5
    3x +4z = 2
    2y + 3w = -2
    3z - 2w = 1

    from the 1st equation, x = 5-2y
    sub into 2nd equation ...
    3(5-2y) + 4z = 2

    from 4th equation, w = (3z-1)/2
    sub into 3rd equation...
    2y + 3[(3z-1)/2] = -2

    now you have two equations with y and z as variables...
    3(5-2y) + 4z = 2 ... 2y + 3[(3z-1)/2] = -2
    15 - 6y + 4z = 2 ... 4y + 3(3z-1) = -4
    -6y + 4z = -13 ... 4y + 9z - 3 = -4
    ... 4y + 9z = -1

    4(-6y + 4z = -13)
    6(4y + 9z = -1)

    -24y + 16z = -52
    24y + 54z = -6
    -----------------
    70z = -58
    z = -58/70 = -29/35

    3z - 2w = 1
    -87/35 - 2w = 1
    w = -61/35

    3x + 4z = 2
    3x - 116/35 = 2
    x = 62/35

    x + 2y = 5
    62/35 + 2y = 5
    2y = 5 - 62/35
    y = 113/70

    The person that made up this system was definitely a numerical sadist...some ugly solutions, but they all work.
     
  4. Sep 14, 2003 #3
    Thanks man, The Frustration has left. I might have solved it correctly but my work on the paper was everywhere and unorganized. You brought it out clearly with a method I was familiar with, only i did not use it hehe. Once again thank you.
     
  5. Sep 14, 2003 #4
    Zeronem,
    What method were you using?

    Here’s one;
    If you were allowed to use matrix operations to solve this (or want another way to check your answer with a calculator) you could rewrite it like this;

    x + 2y = 5
    3x +4z = 2
    2y + 3w = -2
    3z - 2w = 1

    becomes;

    0W + 1X + 2Y + 0Z = 5
    0W + 3X + 0Y + 4Z = 2
    3W + 0X + 2Y + 0Z = -2
    -2W + 0X + 0Y + 3Z = 1

    Which becomes;

    0 1 2 0 * W = 5
    0 3 0 4 * X = 2
    3 0 2 0 * Y = -1
    -2 0 0 3 * Z = 1

    (sorry, I don't know how to put brackets into that)
    Which is in the form of;

    Ax = y, or if you prefer; Ax = C


    ...Where you can pre multiply both sides by the inverse of matrix A to solve for column vector x (the one with the W, X, Y, and Z);
    x = A-1y

    This would give the same answers Skeeter showed.
    While it may look rather intimidating if you haven't learned about matrix operations, it is conceptually quite easy, just tedious in practice. However, for a modern calculator it is very fast and straightforward.
     
    Last edited by a moderator: Sep 14, 2003
  6. Sep 14, 2003 #5
    Yes, i actually used to know the Augmented Matrix Method but I forgot it in time.
    Here was the method I was useing.

    x + 2y = 5
    3x + 4z = 2
    2y + 3w = -2
    3z - 2w = 1

    -3(x + 2y)=5(-3)---> -3x - 6y = -15 the 3 and -3 cancel.
    3x + 4z = 2------> 3x + 4z = 2
    = -6y + 4z = -13

    Then i would take
    2y + 3w = -2
    3z - 2w = 1
    make the w's cancel each other
    2(2y + 3w) = -2(2) ------> 4y + 6w = -4
    3(3z - 2w) = 1(3)--------> 9z - 6w = 3
    = 4y + 9z = -1
    .
    Then we take -6y + 4z = -13 and 4y + 9z = -1
    and cancel the y
    2(-6y + 4z) = -13(2) -----------> -12y + 8z = -26
    3(4y + 9z) = -1(3) -------------> 12y + 27z = -3
    35z = -29
    z = -29/35
    This was the method I was using. I think I could of pulled it off if my work wasn't so unorganized on the paper I was using. I usually get completely careless in these problems when the Teacher teaches us stuff that is like kindergarden work for me.
     
    Last edited: Sep 14, 2003
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