Proof: By Euclid
(related to Corollary: Equivalent Statements Regarding Parallel Lines)
The corollary follows immediately from the propositions 1.27, 1.28, and 1.29.
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"