Lemma: Lem. 13.18: Angle of the Pentagon
(Lemma to Proposition 18 from Book 13 of Euclid's “Elements”)
It can be shown that the angle of an equilateral and equiangular pentagon is one and one-fifth of a right angle, as follows.
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Adapted from (subject to copyright, with kind permission)
- Fitzpatrick, Richard: Euclid's "Elements of Geometry"
Adapted from CC BY-SA 3.0 Sources:
- Prime.mover and others: "Pr∞fWiki", https://proofwiki.org/wiki/Main_Page, 2016