Spherical Law of Sines
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 , and be the angles at the vertices , and . The spherical law of sines states that .
Let be the orthogonal projection of onto the plane . Let and be the orthogonal projections of onto the lines and , respectively. Then , , , and , so .
 Wikipedia. "Spherical Law of Cosines." (Feb 22, 2017) en.wikipedia.org/wiki/Spherical_law_of _cosines.
 Wikipedia. "Spherical Trigonometry." (Feb 22, 2017) en.wikipedia.org/wiki/Spherical_trigonometry.