Godfrey, Charles; Siddons, Arthur Warry (1908). Modern Geometry. University Press. p. 20.
doi.org
Ostermann, Alexander; Wanner, Gerhard (2012). "The Theorems of Apollonius–Pappus–Stewart". Geometry by Its History. Springer. § 4.5, pp. 89–91. doi:10.1007/978-3-642-29163-0_4.