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I'm trying to read a proof in Dummit and Foote of the statement

Suppose $K/F$ is a Galois extension and F'/F is any extension. Then KF'/F' is a Galois extension, and Gal(KF'/F') \cong Gal(K/K \cap F').

One line I am confused about is

Since $K/F$ is Galois, every embedding of $K$ fixing $F$ is an automorphism of $K$, so the map $\varphi: Gal(KF'/F') \to Gal(K/F$), $\sigma \mapsto \sigma\vert_K$ defined by restricting an automorphism $\sigma$ to the subfield $K$ is well-defined.

I take it this means automorphisms of KF' fixing F' (thus fixing $F$) also send $K$ to $K$? If that's what it means, why is it true?

Thanks!

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    @JiangweiXue How did you know that the $K/F$ is algebraic because if $K/F$ is Galois and algebraic then only it is normal.2017-03-20

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