The question goes like this: For $x,y,z\in\mathbb{N}$, $x^2+y^2+1=xyz$.
Prove $z = 3$.
How do I even start?
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Very close to (https://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board=riddles_medium;action=display;num=1214032409) – Jean Marie Oct 25 '16 at 13:09
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Looking at the equation modulo $3$ shows that $z$ is divisible by $3$, which is a start. – Arthur Oct 25 '16 at 13:12
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http://www.artofproblemsolving.com/community/c3046h1048219 – individ Oct 25 '16 at 13:18