Prove that if $a$ is irrational then $\sqrt a$ is irrational

Question

Just hints but solution thx. Any hints for me? I simply suppose that

$a = \dfrac mn$

then $\sqrt a = \sqrt{\dfrac mn}$

But this does not make sense ..

Answer 1

Hint: Prove the contrapositive.

Answer 2

I got the solution.

Assuming that, $\sqrt{a} = \frac{n}{m}$ then we have a is $\frac{n^2}{m^2}$. Obviously this is also a rational number.

Hint from @Git Gud.