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Homework Help: 1 equation, 2 unknowns, need integer solution

  1. Sep 10, 2011 #1
    1. The problem statement, all variables and given/known data

    I needed to solve this single equation with two unknowns.

    199x - 98y = -5

    0< x <=99
    0< y <=99

    I typed the equation into Wolfram Alpha and got an integer solution of:

    x = 98n + 31
    y = 199n +63 when n is an integer

    Since I know my restriction on x and y I can conclude that my solution is:

    x = 31
    y = 63 when n = 0

    My question is, how do I obtain that integer solution that Wolfram Alpha gave me?

    [edit, changed the + to a - sign from an error Ray Vickson pointed out, thanks.]
     
    Last edited: Sep 10, 2011
  2. jcsd
  3. Sep 10, 2011 #2

    Ray Vickson

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    There is something wrong with your question. If x and y are integers >= 1, then 199x + 98y is >= 207, so can't be equal to -5.

    RGV
     
  4. Sep 10, 2011 #3
    Sorry, it was supposed to be:

    199x - 98y = -5
     
  5. Sep 11, 2011 #4
    How about solving for y and then graphing it, and looking for where the line crosses two integers?
     
  6. Sep 11, 2011 #5

    phyzguy

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    These are called Diophantine equations:
    http://en.wikipedia.org/wiki/Diophantine_equation#Linear_Diophantine_equations
    I don't think there is a general method for finding solutions - with relatively small numbers like this, the quickest way is to simply exhaustively search through the allowed integers and see if there are any solutions.
     
  7. Sep 11, 2011 #6
    Since the GCD of 99 and 198 is 1, there are integers x and y such that

    99 x + 198 y = 1

    You can find x and y by several methods, such as the Extended Euclidean Algorithm

    http://en.wikipedia.org/wiki/Extended_Euclidean_algorithm

    Then 99 (-5x) + 198 (-5y) = -5

    That gives you one solution, not necessarily in the acceptable range, but maybe you can use that to find others.
     
  8. Sep 12, 2011 #7
    Its a common linear diophantine equation. Go search for it :)
     
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