Hi, can somebody please provide me with a source for a proof/ outline the method:

Let char(F)=0, Suppose f in F[t] be irreducible of prime degree and K/F a splitting field of f.
Then f is solvable by radicals iff. K=F(r_i,r_j) for any two roots r_i and r_j of f in K.


THANKS!
mcasolin