Heine-Borel Theorem/Euclidean Space/Necessary Condition/Proof 1

Theorem
For any natural number $n \ge 1$, a closed and bounded subspace of $\R^n$ is compact.

Proof
Let $C \subseteq \R^n$ be closed and bounded.

Since $C$ is bounded, $C \subseteq \left[{a \,.\,.\, b}\right]^n = B$ for some $a, b \in \R$.

By the main theorem and by Topological Product of Compact Spaces, it follows that $B$ is compact.

By the corollary to Closed Sets in Topological Subspace, we have that $C$ is closed in $B$.

By Closed Subspace of Compact Space is Compact, $C$ is compact.