Maths Olympiad Prep

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Number theory Difficulty 5.8 AIME, harder Prove it Austria

Determine all integer solutions of the equation
(x1)x+(x+1)+(y1)y+(y+1)=249xy. (x - 1) \cdot x + (x + 1) + (y - 1) \cdot y + (y + 1) = 24 - 9 \cdot xy.

Solution

Since (x1)x(x+1)+(y1)y(y+1)=x3+y3xy(x - 1) \cdot x \cdot (x + 1) + (y - 1) \cdot y \cdot (y + 1) = x^3 + y^3 - x - y, adding 3xy(x+y)3xy(x + y) to both sides of the equation yields the equivalent equation
(x+y)3(x+y)=24+3xy(x+y3)    (x+y)327(x+y3)=3xy(x+y3). (x+y)^3 - (x+y) = 24 + 3xy(x+y-3) \iff (x+y)^3 - 27 - (x+y-3) = 3xy(x+y-3).
Since (x+y)327=(x+y3)((x+y)2+3(x+y)+9)(x + y)^3 - 27 = (x + y - 3)((x + y)^2 + 3(x + y) + 9), this is equivalent to
(x+y3)((x+y)2+3(x+y)+913xy)=0    (x+y3)(x2xy+y2+3x+3y+8)=0. (x+y-3)((x+y)^2+3(x+y)+9-1-3xy) = 0 \iff (x+y-3)(x^2-xy+y^2+3x+3y+8) = 0.
If the expression x+y3x + y - 3 is equal to 00, we obtain the set of solutions
{(t,3t); tZ}. \{(t, 3-t);\ t \in \mathbb{Z}\}.
It remains to find all solutions of the equation x2xy+y2+3x+3y+8=0x^2 - xy + y^2 + 3x + 3y + 8 = 0.
If we consider the equivalent equation x2(y3)x+y2+3y+8=0x^2 - (y-3) \cdot x + y^2 + 3y + 8 = 0 as a quadratic equation in xx, the discriminant (y3)24(y2+3y+8)=3y218y23=43(y+3)2(y-3)^2 - 4(y^2 + 3y + 8) = -3y^2 - 18y - 23 = 4 - 3(y+3)^2

must be a perfect square if the solutions are to be integers. This is the case iff (y+3)2=0(y+3)2=1(y+3)^2 = 0 \lor (y+3)^2 = 1, i.e. iff y=2y=3y=4y = -2 \lor y = -3 \lor y = -4.
For y=2y = -2 we obtain the equation x2+5x+6=0x^2 + 5x + 6 = 0 for xx, and thus the solutions (2,2)(-2, -2) and (3,2)(-3, -2).
For y=3y = -3 we obtain the equation x2+6x+8=0x^2 + 6x + 8 = 0 for xx, and thus the solutions (2,3)(-2, -3) and (4,3)(-4, -3).
Finally, for y=4y = -4 we obtain the equation x2+7x+12=0x^2+7x+12=0 for xx, and thus the solutions (3,4)(-3, -4) and (4,4)(-4, -4), completing the set of solutions. \square

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