Proposition: 5.02: Multiplication of Numbers is Right Distributive over Addition
(Proposition 2 from Book 5 of Euclid's “Elements”)
If a first (magnitude) and a third are equal multiples of a second and a fourth (respectively), and a fifth (magnitude) and a sixth (are) also equal multiples of the second and fourth (respectively), then the first (magnitude) and the fifth, being added together, and the third and the sixth, (being added together), will also be equal multiples of the second (magnitude) and the fourth (respectively).
 For let a first (magnitude) $AB$ and a third $DE$ be equal multiples of a second $C$ and a fourth $F$ (respectively).
 And let a fifth (magnitude) $BG$ and a sixth $EH$ also be (other) equal multiples of the second $C$ and the fourth $F$ (respectively).
 I say that the first (magnitude) and the fifth, being added together, (to give) $AG$, and the third (magnitude) and the sixth, (being added together, to give) $DH$, will also be equal multiples of the second (magnitude) $C$ and the fourth $F$ (respectively).
Modern Formulation
If we are given two positive real numbers \(\alpha\), \(\beta\),^{1} and the following multiples of aliquot parts \(m > 1\), \(n > 1\):
\[m\alpha=m\beta,\quad n\alpha=n\beta\quad\quad( * )\]
then adding both equations gives us
\[(m+n)\alpha=m\beta+n\beta.\]
General Modern Formulation
See distributivity law for real numbers.
Table of Contents
Proofs: 1
Mentioned in:
Proofs: 1 2 3
Sections: 4
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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
Footnotes