The Thirteen Books of the Elements. Volume 1. Books 1-2. Translated from the text of Heiberg. By Thomas L. Heath

Pythagoras Theorem: In right-angled triangles the square on the side subtending the right angle is equal to the squares on the sides containing the right angle. Euclid, Elements, Volume 1, Book 1-2, Book 1, Proposition 47. p. 349. (also PDF: page 360) See also: archive.org\href{https://archive.org/details/EuclidsElementsBooksIIIVolume1Heath/}{archive.org, San Francisco, CA 94118, USA}} \href{https://doi.org/10.5281/zenodo.5983970}{Zenodo}