Question on proof that closed subset of compact metric space is compact 
Proposition. If $K$ is a compact metric space, $F \subset K$, and $F$ is closed, then $F$ is compact.
Proof. Suppose $F$ is a closed subset of the compact set $K$. If $\mathcal{G} = \{G_\alpha\}$ is an open cover of $F$, then $\mathcal{G}' = \mathcal{G} \cup \{F^c\}$ will be an open cover of $K$. We are given that $K$ is compact, so let $\mathcal{H}$ be a finite subcover of $\mathcal{G}'$ for $K$. If $F^c$ is one of the open sets in $\mathcal{H}$, omit it. The resulting finite subcover of $\mathcal{G}$ will be an open cover of $F$.

I have a question. So I understand that we can throw out $F^c$, as it's disjoint from $F$. But how do we know that a finite subcover of $\mathcal{G}' = \mathcal{G} \cup \{F^c\}$ (which for all purposes and intents, just consider $\mathcal{G}$) for $K$ is actually an open cover of $F$? How do we know we aren't missing any part of $F$, i.e. there's a part of $F$ that hasn't been covered?

Basically what this proof does is it shows that given an open cover $U$ of $F$, we can extend $U$ to cover all of $K$ by adding the open set $F^c$ (this is possible since $F$ is closed). Then since this new open cover  covers all of $K$, it must have a finite subcover by compactness, call it $U' \subseteq U \cup \left\{F^c \right\}$. Then $U'$ covers $F$ since $F \subseteq K$, and we can toss out $F^c$ to get an open cover of $F$ that is a finite subcover of $U$. Since $U$ was arbitrary, the claim follows.
Note that we didn't use any particular properties of metric spaces here, this actually holds in any compact topological space $K$.
