Relations between Plane Angles and Solid Angles in a Trihedron
Let , and be the edges of a trihedron that determines a solid angle. The plane angles opposite the edges are denoted , , , and the angles between the edges and their opposite faces are denoted , , . Construct three planes parallel to the faces , and at distance 1 from the corresponding faces. Let the intercepts of these planes with edges of the solid angle be , , . Also define the points , , , such that , , , , to get a parallelepiped with all faces of equal area, since all heights are equal. The lengths of the edges are , and . The areas of the faces are , and .[more]
Since the areas are equal, .[less]
 Wikipedia. "Spherical Law of Cosines." (Feb 23, 2017) en.wikipedia.org/wiki/Spherical_law_of _cosines.
 Wikipedia. "Spherical Trigonometry." (Feb 23, 2017) en.wikipedia.org/wiki/Spherical_trigonometry.
 P. R. Cromwell, Polyhedra, New York: Cambridge University Press, 1997.