http://mathworld.i2p/Point-PointDistance3-Dimensional.html
Verifying the arc length gives (18) where (19) (20) See also Calculus of Variations , Circle Triangle Picking , Great Circle , Line Line Picking , Point-Point Distance--2-Dimensional , Point-Quadratic Distance , Sphere Point Picking References Arfken, G. Mathematical Methods for Physicists, 3rd ed.