Proof: By Euclid
(related to Corollary: Parallelogram - Defining Property II)
This follows immediately from Prop 1.34 and the definition of a parallelogram or a rhombus.
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"