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Number theory Difficulty 5.3 AIME, harder Prove it Saudi Arabia

Find all triples (x,y,z)(x, y, z) of integers such that
x2y+y2z+z2x=20102 and xy2+yz2+zx2=2010. x^{2} y + y^{2} z + z^{2} x = 2010^{2} \text{ and } x y^{2} + y z^{2} + z x^{2} = -2010.

Solution

We replace 20102010 by p1p-1, for some odd prime. Subtracting the first equation from the second, we obtain
(xy)(yz)(zx)=p(p1) (x-y)(y-z)(z-x) = -p(p-1)
We have (xy)+(yz)+(zx)=0(x-y)+(y-z)+(z-x)=0 and (xy)(yz)(zx)<0(x-y)(y-z)(z-x)<0, so precisely two of them are positive. Assume that xy>0x-y>0 and yz>0y-z>0. Without loss of generality, suppose xyyzx-y \leq y-z. Because pp is a prime, the only possibility is
xy=1,yz=p1,zx=p x-y=1, \quad y-z=p-1, \quad z-x=-p
Then x=y+1x=y+1, z=y(p1)z=y-(p-1), and the first equation reduces to
y[3y2+3(p2)y+(p2)2]=0 y\left[3y^{2} + 3(p-2)y + (p-2)^{2}\right]=0
The only solution is y=0y=0, implying x=1x=1 and z=p1z=p-1.
The solutions (x,y,z)(x, y, z) are (1,0,1p)(1,0,1-p), (1p,1,0)(1-p, 1,0) and (0,1p,1)(0,1-p, 1). We have p=2011p=2011, hence the desired triples are (1,0,2010)(1,0,-2010), (2010,1,0)(-2010,1,0), and (0,2010,1)(0,-2010,1).

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