(related to Corollary: Angles of a Right And Isosceles Triangle)

- By Prop 1.32, the sum of all angles in a plane triangle is $180^\circ.$
- If a right triangle is isosceles, then their angles are half of $180^\circ-90^\circ=90^\circ,$ i.e. a right angle.∎

**Callahan, Daniel**: "Euclidâ€™s 'Elements' Redux" 2014

**Casey, John**: "The First Six Books of the Elements of Euclid"