John Casey: A sequel to the first six books of the Elements of Euclid, containing an easy introduction to modern geometry, with numerous examples. Hodges, Figgis & co., Dublin 1886, S.34 (archive.org – Prop. 12, Cor. 1).
idoc.pub
R. A. Johnson: Advanced Euclidean Geometry. An Elementary Treatise on the Geometry of the Triangle and the Circle. Dover Publications, New York 1960, ISBN 978-0-486-15498-5, S.71 (idoc.pub).
