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I A nonlinear equation system

  1. Mar 20, 2016 #1
    I'm trying to find a closed form (an algebraic solution) for the following system:

    x² - y² = 5
    x + y = xy

    It's a bit tricky but I manage to end up with the quartic equation:
    x^4 - 2x^3 + 5x^2 -10x + 5 =0
    And this is where I get stuck looking for a closed form root.
    Any suggestion would be appreciated
  2. jcsd
  3. Mar 20, 2016 #2
    trial and error of synthetic division.
  4. Mar 21, 2016 #3


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    Staff Emeritus
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    You won't find any integer or rational solutions. The general solution to a fourth degree equation is pretty daunting.


    This particular equation has two real and a pair of complex-conjugate roots.

    BTW, I checked your algebra in reducing your system of equations to one equation in x. I think you have some mistakes there, since I don't obtain your particular quartic equation.

    In any event, the resulting quartic still has two real and a pair of complex-conjugate solutions, none of which are nice integers or rationals.

    I used Wolfram Alpha to solve for the roots. It's much easier than anything else.
  5. Mar 21, 2016 #4
    Indeed, the right resulting quartic equation is x^4 - 2x^3 - 5x^2 -10x - 5 =0. I used Cardano and Lagrange method to find the real roots, but their form is very ugly.
    Actually I got this equation while trying to solve a simple geometrical problem. I'll post it today on a new thread, I'm interested in finding a simpler method to solving it since mine leads to a quartic equation.

    Thanks for the contribution.
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