IMO 1987 LL GBR23

Origin: GBR

Problem

A lampshade is part of the surface of a right circular cone whose axis is vertical. Its upper and lower edges are two horizontal circles. Two points are selected on the upper smaller circle and four points on the lower larger circle. Each of these six points has three of the others that are its nearest neighbors at a distance d from it. By distance is meant the shortest distance measured over the curved survace of the lampshade. Prove that the area of the lampshade if d2(2\theta + \sqrt 3), where cot \theta 2 = 3 \theta.