First, we know that $E$ is the splitting field of some polynomial over $F$, so there exists a polynomial $f(x) \in F[x]$ such that $E = F(\alpha_1, \alpha_2, \ldots, \alpha_n)$, where $\alpha_1, \alpha_2, \ldots, \alpha_n$ are the roots of $f(x)$ in $E$.
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