Maths Olympiad Prep

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Algebra Difficulty 5.5 AIME, harder Prove it Saudi Arabia

Find all triples (x,y,z)(x, y, z) of real numbers such that
x2+y2+z2+1=xy+yz+zx+x2y+z. x^{2}+y^{2}+z^{2}+1=xy+yz+zx+|x-2y+z| .

Solution

We can write
x2+y2+z2+1=xy+yz+zx+xy+zy x^{2}+y^{2}+z^{2}+1=xy+yz+zx+|x-y+z-y|
hence
(xy)2+(yz)2+(zx)2+2=2xy+zy. (x-y)^{2}+(y-z)^{2}+(z-x)^{2}+2=2|x-y+z-y| .
It follows
(xy)2+(yz)2+(zx)2+22xy+2yz. (x-y)^{2}+(y-z)^{2}+(z-x)^{2}+2 \leq 2|x-y|+2|y-z| .
The last relation is equivalent to
(xy1)2+(yz1)2+(zx)20. (|x-y|-1)^{2}+(|y-z|-1)^{2}+(z-x)^{2} \leq 0 .
We get xy=1|x-y|=1, yz=1|y-z|=1 and x=zx=z. The desired triples (x,y,z)(x, y, z) are (a,a1,a)(a, a-1, a), (a,a+1,a)(a, a+1, a), where aRa \in \mathbb{R}.

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