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Number theory Difficulty 5.5 AIME, harder Prove it Bulgaria

Find all positive integers xx and yy such that xy2+2yxy^2 + 2y divides 2x2y+xy2+8x2x^2y + xy^2 + 8x.

Solution

Since xy2+2yxy^2 + 2y divides
(2x+y)(xy2+2y)y(2x2y+xy2+8x)=2y24xy,(2x + y)(xy^2 + 2y) - y(2x^2y + xy^2 + 8x) = 2y^2 - 4xy,
we conclude that xy+2xy + 2 divides 2y4x2y - 4x. We consider two cases.

Case 1. Let 2y4x02y - 4x \ge 0. Then we have two possibilities:

1.1) If x2x \ge 2 then xy+2>2y4xxy + 2 > 2y - 4x. Hence 2y4x=02y - 4x = 0, i.e. x=ax = a and y=2ay = 2a, where a2a \ge 2 is an integer. Since xy2+2y=4a(a2+1)xy^2 + 2y = 4a(a^2 + 1) divides 2x2y+xy2+8x=8a(a2+1)2x^2y + xy^2 + 8x = 8a(a^2 + 1), this gives a solution.

1.2) If x=1x = 1 then y2+2yy^2 + 2y divides 88, i.e. y=2y = 2.

Hence in this case the solutions are (a,2a)(a, 2a), where aa is a positive integer.

Case 2. Let 2y4x<02y - 4x < 0, i.e. 4x2y>04x - 2y > 0. If y4y \ge 4, then xy+2>4x2xy + 2 > 4x - 2. Therefore y=1,2y = 1, 2 or 33.

2.1) If y=1y = 1 then 2x2+9xx+2=2x+510x+2\frac{2x^2 + 9x}{x + 2} = 2x + 5 - \frac{10}{x + 2} is an integer. Hence x+2x + 2 divides 1010 and this gives the solutions x=3,y=1x = 3, y = 1 and x=8,y=1x = 8, y = 1.

2.2) If y=2y = 2 then x2+3xx+1=x+22x+1\frac{x^2 + 3x}{x + 1} = x + 2 - \frac{2}{x + 1} is an integer which gives x=1x = 1.

2.3) If y=3y = 3 then 6x2+17x9x+6\frac{6x^2 + 17x}{9x + 6} is an integer. This implies that 3x3|x, i.e. x=3kx = 3k for some positive integer kk. After simplifications we obtain that 18k2+17k9k+2=(2k+1)+4k29k+2\frac{18k^2 + 17k}{9k + 2} = (2k + 1) + \frac{4k - 2}{9k + 2} is an integer, which is impossible for k1k \ge 1.

Finally, the solutions are x=a,y=2ax = a, y = 2a for all positive integers aa and x=3,y=1,x=8,y=1x = 3, y = 1, x = 8, y = 1.

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.