Sykes, Mabel; Comstock, Clarence E. (1922), Solid Geometry, Rand McNally & Company, p. 49 https://archive.org/details/solidgeometry00sykerich/page/48 ↩
Slaught, H. E.; Lennes, N. J. (1911), Solid Geometry with Problems and Applications, Allyn & Bacon, p. 34 https://archive.org/details/solidgeometrywit00slaurich/page/32 ↩