Proof: By Euclid
(related to Corollary: Angles of Right Triangle)
- By Prop 1.32, the sum of all angles in a plane triangle is $\alpha+\beta+\gamma=180^\circ.$
- Without loss of generality, let $\alpha+\gamma=\beta.$
- Then $2\beta=180^\circ,$ and $\beta=90$ is a right angle.
Thank you to the contributors under CC BY-SA 4.0!
Adapted from CC BY-SA 3.0 Sources:
- Callahan, Daniel: "Euclid’s 'Elements' Redux" 2014
Adapted from (Public Domain)
- Casey, John: "The First Six Books of the Elements of Euclid"