(a) Show that $\Pi _ { \alpha }$ is irreducible in $\mathbb { Q } [ X ]$.\\
(b) Let $P \in \mathbb { Q } [ X ]$ be a monic polynomial, irreducible in $\mathbb { Q } [ X ]$. Show that if $z$ is a complex root of $P$, then $P$ is the minimal polynomial of $z$.