Proposition: Prop. 9.21: Sum of Even Numbers is Even
(Proposition 21 from Book 9 of Euclid's “Elements”)
If any multitude whatsoever of even numbers is added together then the whole is even.
* For let any multitude whatsoever of even numbers, $AB$, $BC$, $CD$, $DE$, lie together.
* I say that the whole, $AE$, is even.
Modern Formulation
(not yet contributed)
Table of Contents
Proofs: 1
Mentioned in:
Proofs: 1 2 3
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