Question 7
Let $x$, $y$, $n$ be positive integers such that
$$
n = \frac{x^2 - 1}{2} = \frac{y^2 - 1}{3}.
$$
Prove that $n = y^2 - x^2$ and $20$ divides $n$.
$$
n = \frac{x^2 - 1}{2} = \frac{y^2 - 1}{3}.
$$
Prove that $n = y^2 - x^2$ and $20$ divides $n$.