Let $X$ consist of points on the surface of a sphere. Define a distance function $d(P,Q)$ as the length of the arc of the great circle passing through points $P$ and $Q$ on the surface of the sphere. Show that $d$ satisfies the triangle inequality.
I'm not sure how to get started. One way could be to express the arc lengths (say between $P$ and $Q$) in terms of the angle between the lines joining the center of the sphere to $P$ and $Q$. So if $\theta_1$ is the angle between $P$ and $Q$, $\theta_2$ is the angle between $Q$ and $R$, and $\theta_3$ is the angle between $P$ and $R$, then we'd effectively have to prove that $\theta_i + \theta_j \geq \theta_k$.
Beyond this, I'm at a loss on how to proceed, or whether this is even the correct method to go about.