Maths Olympiad Prep

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Number theory Difficulty 5.6 AIME, harder Prove it Croatia

Determine all pairs (x,y)(x, y) of integers such that
y2=x3+3x2+2x. y^2 = x^3 + 3x^2 + 2x.

Solution

The factorization of the right hand side of the equation gives us
y2=x(x+1)(x+2). y^2 = x(x + 1)(x + 2).
Since xZx \in \mathbb{Z}, obtained factors are consecutive integers.
First we assume that all the factors, xx, x+1x+1 and x+2x+2 are different from zero. Since the left hand side of the equality is positive, these three factors also have to be positive.
It follows that x+1x+1 and x(x+2)x(x+2) are relatively prime (since x+1x+1 and xx are relatively prime and also x+1x+1 and x+2x+2 are relatively prime). The product of two relatively prime positive integers is a perfect square if and only if each of the numbers is a perfect square. In particular, that means that
x(x+2)=x2+2x=(x+1)21 x(x + 2) = x^2 + 2x = (x + 1)^2 - 1
is a perfect square. Therefore, (x+1)21(x+1)^2 - 1 and (x+1)2(x+1)^2 are two consecutive integers which are both perfect squares, and that is possible if and only if those numbers are zero and one. Hence,
(x+1)2=1 (x+1)^2 = 1
so x=2x = -2 or x=0x = 0. But then x+2=0x + 2 = 0 or x=0x = 0, which is a contradiction with the assumption that all the factors are different from zero.
If one of the factors on the right hand side is equal to zero, then y=0y = 0 and we get the solutions
(x,y){(2,0),(1,0),(0,0)}. (x, y) \in \{(-2, 0), (-1, 0), (0, 0)\}.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.