MHB Infinitely Many Pairs: Integer Roots of Quadratic Equations

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SUMMARY

The discussion focuses on demonstrating that there are infinitely many pairs \((a, b)\) such that both quadratic equations \(x^2 + ax + b = 0\) and \(x^2 + 2ax + b = 0\) have integer roots. An integer root occurs when \(\frac{-a \pm \sqrt{a^2 - 4b}}{2}\) is an integer, leading to the conclusion that the conditions for \(a\) and \(b\) must be established to ensure integer roots for both equations. The participants emphasize deriving a condition on \(a\) and \(b\) that satisfies both equations simultaneously, ultimately proving the existence of infinitely many valid pairs.

PREREQUISITES
  • Understanding of quadratic equations and their roots
  • Familiarity with the discriminant \((a^2 - 4b)\)
  • Basic knowledge of integer properties in mathematics
  • Ability to manipulate algebraic expressions
NEXT STEPS
  • Research the properties of integer roots in quadratic equations
  • Study the implications of the discriminant on root types
  • Explore conditions for integer solutions in polynomial equations
  • Investigate the relationship between coefficients and roots in quadratic forms
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Mathematicians, educators, and students interested in number theory, particularly those exploring integer solutions in polynomial equations.

juantheron
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how can we show that there are infinitely many pairs $(a,b)$ such that both the quadratic equations -

$x^2 + ax +b = 0$ and $x^2 +2ax +b = 0$ have integer roots
 
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Re: infinity many pairs

I'll get you started. What is an integer root? A quadratic equation x^2 + ax + b = 0 yields integer roots when \frac{-a \pm \sqrt{a^2 - 4b}}{2} is an integer, with a, b \in \mathbb{Z} (a and b don't need to be relative numbers but if we can prove there are infinitely many pairs of relative numbers \left (a, b \right ) we don't need to worry about non-integers).

Clearly if \frac{-a \pm \sqrt{a^2 - 4b}}{2} is an integer, then -a \pm \sqrt{a^2 - 4b} is one too.

Can you follow this reasoning to the end and establish a condition on a and b such that the resulting quadratic has integer roots? Then, do the same for the other equation with 2a instead and put all the conditions together. Then, prove that all these conditions are satisfied for infinitely many pairs \left (a, b \right ) and you will be done. Does that make sense?
 

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