<s>[INST] How to prove infinitely many integer triples $x,y,z$ such that $x^2 + y^2 + z^2$ is divisible by $(x + y +z)$ I'm working on a problem where we must show that there are infinitely many integer triples $x,y,z$ such that $x^2 + y^2 + z^2$ is divisible by $(x + y +z)$ and $x,y,z$ are pairwise coprime. Also $x,y,z$ are distinct.
I can see that either all $x,y,z$ are odd or that one of them is even but besides that haven't made much progress (I can't see any nice factorization of $x^2 + y^2 + z^2$ nor have I been able to find an easy example to get a foothold). 
Broadly I'm thinking that my two major approaches are proof by contradiction that the number of triples can't be finite, or trying to find some construction to generate the triples.
I imagine finding a construction would be more fruitful.
I would appreciate both a solution but also advice on how to approach this type of problem
 [/INST] Take $(x, y, z) = (3k, 3k + 1, 54k^2 + 12k + 1)$ for arbitrary $k \in \mathbb{N}_+$. Since$$
(x, y) = (3k, 3k + 1) = 1,
$$\begin{align*}
(x, z) &= (3k, 54k^2 + 12k + 1)\\
&= (3k, 54k^2 + 12k + 1 - 3k(18k + 4)) = (3k, 1) = 1,
\end{align*}\begin{align*}
(y, z) &= (3k + 1, 54k^2 + 12k + 1)\\
&= (3k + 1, 54k^2 + 12k + 1 - (3k + 1)(18k - 2)) = (3k + 1, 3) = 1,
\end{align*}
then $x, y, z$ are pairwise coprime. Note that $x + y + z = 54k^2 + 18k + 2$ and\begin{align*}
x^2 + y^2 + z^2 &= (3k)^2 + (3k + 1)^2 + (54k^2 + 12k + 1)^2\\
&= 2916k^4 + 1296k^3 + 270k^2 + 30k + 2\\
&= (54k^2 + 18k + 2)(54k^2 + 6k + 1),
\end{align*}
thus $x+ y +z \mid x^2 + y^2 + z^2$.
</s>