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System of equations : Help

  1. Sep 14, 2009 #1
    I have been trying to solve this problem for a whole week without success, I need help.

    Solve the following for x,y and z:

    2x+5y-z = 18
    7x-y+4z = 22
    6x+2y-3z = 0.1x + 0.2y + 0.3z

    I have already combined all the terms of eq.#3 and equal it to zero, but it didn't work.
    I also, multiplyed eq.#3 by 1 and tried to have a comon x for the other two equations to eliminate x and it didn't work, I've tried many other tatics but it doesn't work.

    please help
    Thank you
  2. jcsd
  3. Sep 15, 2009 #2


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    Code (Text):
    Code (Text):
    Code (Text):
    linsolve([2 5 -1; 7 -1 4; 6-.1 2-.2 -3-.3], [18; 22; 0])
    Code (Text):
  4. Sep 15, 2009 #3
    Maybe you can show some steps or explain in more detail what your problem is? Show us where you're getting stuck or explain what "it didn't work" means.

  5. Sep 15, 2009 #4


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    2x+ 5y- z= 18
    7x- y+ 4z= 22
    5.9x+ 1.8y- 3.3z= 0 ?

    Why in the world would you multiply anything by "1"? Do you mean "10"? That would give you 59x+ 18y- 33z= 0. Getting a "common x" for the other two equations (I guess you mean the same coefficient) would give 14x+ 35y- 7z= 119 and 14x- 5y+ 20z= 110. Subtracting the second equation from the first gives 40y- 27z= 9. What do you mean "it doesn't work"?

    The equations above are the
  6. Sep 15, 2009 #5
    Ahhh - I read it the same way HallsOfIvy did the first time - perhaps you multiplied eq. 3 by eq. 1? Maybe this is the problem - you don't want to multiply, you want to add. Really, you should multiply an equation by a carefully selected constant, then add it to another equation. For example, if you multiply eq. 1 by 4, and add it to eq. 2, you get

    [tex]8x + 20y - 4z + 7x - y +4z = 72 + 22[/tex]
    [tex]15x + 19y = 94[/tex]

    Does this help get you started?

    Last edited: Sep 15, 2009
  7. Sep 16, 2009 #6


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  8. Sep 17, 2009 #7
    Thank you everyone,
    I was confused, but with your help found the solution to this problem
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