Help proving a theorem in my textbook

Question

If $r \in \mathbb{N}$ is not a perfect square, then $\sqrt{r}$ is irrational.

For reference, an integer $n$ is a perfect square if $n=m^2$ for some $m \in \mathbb{Z}$.

Any help proving this would be greatly appreciated.

Answer 1

Hint: Prove by contradiction. Assume $\sqrt r$ is rational, then $\sqrt r=\frac pq$, for some coprime $p,q$. Then $\frac {p^2}{q^2}=r $, but...