Various geometric demonstrations
Euler says that a theorem of Fermat's can be proven in a strictly Euclidean way. He also presents relations about triangles and quadrilaterals, relating products of side lengths, squares of side lengths, and areas.
Original Source Citation
Novi Commentarii academiae scientiarum Petropolitanae, Volume 1, pp. 49-66.
Opera Omnia Citation
Series 1, Volume 26, pp.15-32.