The Thirteen Books of the Elements. Volume 2. Books 3-9. Translated from the text of Heiberg. By Thomas L. Heath

Euclid's theorem: If in a right-angled triangle a perpendicular be drawn from the right angle to the base, the triangles adjoining the perpendicular are similar both to the whole and to one another. Euclid, Elements, Book VI, Proposition 8. p. 209. (also PDF: page 218) see also: archive.org\href{https://archive.org/details/thirteenbooksele00heat_069/}{archive.org, San Francisco, CA 94118, USA}} \href{https://doi.org/10.5281/zenodo.5984056}{Zenodo}