Understanding the inductive proof about field extension in Artin

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This is a proposition in Artin's algebra. But I cannot understand the proof of (b) clearly. Could someone tell me how does he jump from the third step to the fourth step? Thanks so much![enter image description here]1

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$F(a_1,...,a_{k-1})=F'$ is a field. Thus $F(a_1,...,a_{k-1})[a_k]=F'[a_k]$ and you can reapply a) slightly modified with $F'$ instead of $F$