Possible Duplicate:
Alternative proof that $(a^2+b^2)/(ab+1)$ is a square when it's an integer.
I've tried many different approaches and they don't seem to lead anywhere.
The question goes like this :
Let $a$ and $b$ be positive integers ($> 0$) such that $(1+ab) \mid (a^2 + b^2)$. Show that the integer $(a^2 + b^2)/(1+ab)$ must be a perfect square.
