(related to Corollary: Bisectors of Two Supplemental Angles Are Right Angle To Each Other)

- Because the sum of two supplemental angles equals two right angles, their sum is $180^\circ.$
- If we bisect them then each angle is halved into two bisectors, $45^\circ$ each.
- Thus, the sum of two bisectors is $90^\circ$, which is a right angle.∎

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

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