Proposition: 3.21: Angles in Same Segment of Circle are Equal
(Proposition 21 from Book 3 of Euclid's “Elements”)
In a circle, angles in the same segment are equal to one another.
* Let $ABCD$ be a circle, and let $BAD$ and $BED$ be angles in the same segment $BAED$.
* I say that angles $BAD$ and $BED$ are equal to one another.
Modern Formulation
The inscribed angles joining the ends of the same arc are equal.
Table of Contents
Proofs: 1
Mentioned in:
Proofs: 1
Sections: 2
Thank you to the contributors under CC BYSA 4.0!
 Github:

 nonGithub:
 @Fitzpatrick
References
Adapted from (subject to copyright, with kind permission)
 Fitzpatrick, Richard: Euclid's "Elements of Geometry"
Adapted from CC BYSA 3.0 Sources:
 Prime.mover and others: "Pr∞fWiki", https://proofwiki.org/wiki/Main_Page, 2016