For which natural numbers $n$ is $\sqrt n$ irrational? How would you prove your answer? Can someone help me out here? I'm trying to prove this the same way as whether if $\sqrt 2$ is irrational, but i'm not sure what am i doing. 
Can someone show me how to prove this?
In fact, every integer which is not a square has an irrational square root. Can you see how to generalize the result for $\sqrt{2}$ to prove this?
Take $\sqrt a=\frac{p}{q}$ with gcd(p,q)=1 and a is prime square both sides...
$$
a=\frac{p^2}{q^2}
$$
then
$$
2q^2=p^2
$$
then 
$$
a|p^2
$$
a contradiction

then think about what would happen if a was not prime