Kells, Lyman M.; Kern, Willis F.; Bland, James R. (1940). Plane And Spherical Trigonometry (بالإنجليزية). McGraw Hill Book Company, Inc. pp. 323-326. Archived from the original on 2021-05-07. Retrieved 2018-07-13.
Kells, Lyman M.; Kern, Willis F.; Bland, James R. (1940). Plane And Spherical Trigonometry (بالإنجليزية). McGraw Hill Book Company, Inc. pp. 323-326. Archived from the original on 2021-05-07. Retrieved 2018-07-13.