Sines of the Dihedral Angles of a Tetrahedron
Let be a tetrahedron with dihedral angles , , , , , . A theorem relating the sines of the dihedral angles states that .[more]
Here , and are pairs of opposite edges of the tetrahedron.[less]
A proof is described in [1, pp. 111].
 V. V. Prasolov and I. F. Scharjagin, Problems in Stereometry (Russian), Moscow: Nauka, 1989.