Proof that lf K[x] is monic irreducible, deg 2, and has all its

Proof that lf ∫ ϵ K[x] is monic irreducible, deg  ∫≥ 2, and ∫ has all its roots equal (in a splitting field), then char K = p ≠ 0 and ∫ = x^Pn - a for some n ≥ 1 and a ϵ K.