Perimeter Variance of Uniform Random Triangles

Let T be a random triangle in a disk D of radius R (meaning that vertices are independent and uniform in D). We determine the bivariate density for two arbitrary sides a,b of T. In particular, we compute that E(a*b)=(0.837…)*R^2, which implies that Var(perimeter)=(0.649…)*R^2. No closed-form expression for either coefficient is known. The Catalan numbers also arise here.