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Homework Help: Solve the system x+y=9, x^2 - y^2 = 36 for x and y

  1. Sep 12, 2005 #1
    Hi, i just sat my maths exam, and i had a very strange question.... i dont know how to do it...

    x+y=8
    x^2-y^2=36

    find the values for x and y

    Thanks
     
  2. jcsd
  3. Sep 12, 2005 #2
    y = 8-x

    x^2 - (8-x)^2 = 36

    x^2 - (64-16x+x^2) = 36

    16x - 64 = 36
     
  4. Sep 12, 2005 #3
    is that it?

    i did y^2-x^2=36
    (x-y)(x+y)=36
    (x-y)(8)=36
    x-y=36/8

    is any of that correct?
     
  5. Sep 13, 2005 #4

    HallsofIvy

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    Science Advisor

    According to Whozum, 16x- 64= 36 so x= 100/16= 25/4= 6.25 and y= 8- 6.25= 1.75.

    x-y= 6.25- 1.75= 4.5 and 36/8= 9/2= 4.5.

    Your statement is correct but is not a solution to the "problem" which, I suppose, was to solve the two equations.

    In fact, the only thing strange I see about your "question" is that there was no "question"! Are you sure you didn't leave something out- like "solve this pair of equations" or "what are x and y"?
     
  6. Sep 13, 2005 #5

    VietDao29

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    Homework Helper

    Yup. This is correct. Since you have 2 unknowns, you need 2 equations. And you have already had 2 equations. You can then solve:
    [tex]\left\{ \begin{array}{l}x + y = 8 \\ x - y = \frac{9}{2} \end{array} \right.[/tex]
    for x, and y.
    Viet Dao,
     
  7. Sep 13, 2005 #6
    Me or him?
     
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