Solving a Tricky Nonlinear Equation System: A Quest for Closed Form Solutions

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Discussion Overview

The discussion revolves around finding a closed form solution for a nonlinear system of equations represented by x² - y² = 5 and x + y = xy. Participants explore the derivation of a quartic equation and the challenges associated with solving it.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents a quartic equation x^4 - 2x^3 + 5x^2 -10x + 5 = 0 derived from the original system, expressing difficulty in finding closed form roots.
  • Another participant suggests using trial and error with synthetic division as a potential approach.
  • Some participants assert that there are no integer or rational solutions to the quartic equation, noting the complexity of solving fourth degree equations.
  • One participant claims that the correct quartic equation is x^4 - 2x^3 - 5x^2 -10x - 5 = 0, indicating that they used Cardano and Lagrange methods to find real roots, but describe the results as "ugly."
  • Several participants mention the existence of two real roots and a pair of complex-conjugate roots in the quartic equation.
  • One participant references using Wolfram Alpha as a simpler method to find the roots compared to manual calculations.
  • There is a suggestion that mistakes may have been made in the algebraic reduction to the quartic equation, as some participants do not arrive at the same equation.

Areas of Agreement / Disagreement

Participants express differing views on the correctness of the derived quartic equation and the methods to solve it. There is no consensus on the algebraic manipulations or the existence of simpler solutions, indicating ongoing disagreement and uncertainty.

Contextual Notes

Participants note the complexity of the quartic equation and the challenges in finding closed form solutions, with some suggesting that the algebraic steps may contain errors. The discussion reflects a range of approaches and interpretations without resolving the discrepancies.

n7imo
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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
 
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trial and error of synthetic division.
 
n7imo said:
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
You won't find any integer or rational solutions. The general solution to a fourth degree equation is pretty daunting.

https://en.wikipedia.org/wiki/Quartic_function

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

https://en.wikipedia.org/wiki/Quartic_function

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.

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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