# $f=X^p-a\in F[X]$ is irreducible iff $f$ has no root in $F$ [duplicate]

Let $F$ be a field, $a$ an element of $F$ and $p$ prime.
How do I prove that

$f=X^p-a\in F[X]$ is irreducible iff $f$ has no root in $F$?

Honestly, I have no idea how to approach this. Maybe somebody can give me a push in the right direction?

• Notice that one direction is easy : irreducible $\implies$ no root (in $F$). – Watson Mar 26 '16 at 13:31
• – Watson Mar 26 '16 at 13:33
• Also note that this depends on $p$ being prime. For example, $X^4+1$ has no root in $\mathbb{R}$, but it is reducible. – hardmath Mar 26 '16 at 14:50