If $Aut_K(L)$ operates transitive over the zeros of $f$, then $f$ is irreducible

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Let $K$ be a field, $f\in K[X]$ a separable polynomial and $L$ a splitting field of $f$.

Show that if $Aut_K(L)$ operates transitive over the zeros of f, then f is irreducible.

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Hint: supose $\;a_1,...,a_k\;$ are the roots of $\;f(x)=g(x)h(x)\;$ , and suppose $\;a_1,a_2\;$ are roots of the different non-constant factors $\;g(x)\,,\,\,h(x)\;$ .

Is it possible that $\;\sigma a_1=a_2\;$ , for some $\;\sigma\in\text{Aut}_KL\;$ , as given in the problem?