Spherical Law of Cosines
Draw a spherical triangle on the surface of the unit sphere with center at the origin . Let the sides (arcs) opposite the vertices have lengths , and , and let be the angle at vertex . The spherical law of cosines is then given by , with two analogs obtained by permutations.
Let be the plane tangent to the sphere at , and let and . Then , , and . Express the length of in two ways using the usual planar law of cosines for the triangle in the plane :
With the triangle , the law of cosines gives
The two equalities give
 Wikipedia. "Spherical Law of Cosines." (Feb 22, 2017) en.wikipedia.org/wiki/Spherical_law_of _cosines.