Proof by contradiction. Assume that the line segment between points \(P,Q\) has maximum pairwise distance, and that \(Q\) does not lie on a vertex. Let the point on the boundary of our hull found by extending the line \(PQ\) be denoted \(Q'\). This boundary segment is defined between two vertices on our convex hull which we refer to as \(A\) and \(B\). See \(\mathbf{Fig. 1}\) for clarification.

Clearly \(|PQ'|>|PQ|\). In the following section we show that there is always a line-segment longer than \(|PQ'|\). By the transitive property of inequality this segment must also be longer than \(PQ\), a contradiction.

In this case, \(Q=Q^{\prime}\). Observe the angles \(\angle PQ^{\prime}A\) and \(\angle PQ^{\prime}B\). WLOG assume \(\angle PQ^{\prime}B\) is the larger of the two and denote it \(\alpha\).

\begin{align}
\angle PQ^{\prime}A+\angle PQ^{\prime}B & =\pi\notag \\
\therefore\alpha:=\max\left(PQ^{\prime}A,PQ^{\prime}B\right) & \geq\frac{\pi}{2}\notag \\
\end{align}

As such \(\alpha\) must be the unique largest internal angle in the triangle \(PQ^{\prime}B\). Consequently the side opposite \(\alpha\) \(\left(PB\right)\) is the unique hypotenuse of the triangle. As such \(|PB|>|PQ^{\prime}|\). However \(P\) and \(B\) are both elements of the convex hull so we have found a line segment with length larger than our assumed maximum, a contradiction.