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
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Table of Contents
Proofs: 1
Mentioned in:
Proofs: 1 2 3
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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