Let $D$ be a non-square positive integer. Suppose there are positive integers $a$ and $b$ such that $a^2 − Db^2 = 1$. Show that the Diophantine equation $x^2 − Dy^2 = 1$ has infinitely many integer solutions.
I expressed $a^2 − Db^2$ as $a^2 − Db^2 = (a + b\sqrt D)(a − b\sqrt D)$
I'm not sure how to proceed from here. Any help is appreciated.