Spherical Right Triangles
Draw a spherical triangle on the surface of a unit sphere centered at . Denote the arcs opposite the corresponding vertices as , , . Let , , be the angles at the vertices , , . Suppose , so that these become spherical right triangles. The following relations are then valid:[more]
Let be the orthogonal projection of onto the edge . Let be the orthogonal projection of onto . Then , , but also . Thus .
, , but also . Thus .
 Wikipedia. "Spherical Law of Cosines." (Mar 21, 2017) en.wikipedia.org/wiki/Spherical_law_of _cosines.
 Wikipedia. "Spherical Trigonometry." (Mar 21, 2017) en.wikipedia.org/wiki/Spherical_trigonometry.