Proposition: Prop. 13.07: Equilateral Pentagon is Equiangular if Three Angles are Equal
(Proposition 7 from Book 13 of Euclid's “Elements”)
If three angles, either consecutive or not consecutive, of an equilateral pentagon are equal then the pentagon will be equiangular.
* For let three angles of the equilateral pentagon $ABCDE$  first of all, the consecutive (angles) at $A$, $B$, and $C$  be equal to one another.
* I say that pentagon $ABCDE$ is equiangular.
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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