Prove that the orthogonal complement $W^{\perp}$ of a subspace $W \subset V$ is itself a subspace.
$\dagger$ In general, a subset $W \subset V$ of a normed vector space is dense if, for every $\mathbf{v} \in V$, and every $\varepsilon>0$, one can find $\mathbf{w} \in W$ with $\|\mathbf{v}-\mathbf{w}\|<\varepsilon$. The Weierstrass Approximation Theorem, [19; Theorem 10.2.2], tells us that the polynomials form a dense subspace of the space of continuous functions, and underlies the proof of the result mentioned in the preceding paragraph.