Proof: By Euclid
(related to Proposition: 7.16: Natural Number Multiplication is Commutative)
- For since A has made C (by) multiplying B, B thus measures C according to the units in A [Def. 7.15] .
- And the unit E also measures the number A according to the units in it.
- Thus, the unit E measures the number A as many times as B (measures) C.
- Thus, alternately, the unit E measures the number B as many times as A (measures) C [Prop. 7.15].
- Again, since B has made D (by) multiplying A, A thus measures D according to the units in B [Def. 7.15] .
- And the unit E also measures B according to the units in it.
- Thus, the unit E measures the number B as many times as A (measures) D.
- And the unit E was measuring the number B as many times as A (measures) C.
- Thus, A measures each of C and D an equal number of times.
- Thus, C is equal to D.
- (Which is) the very thing it was required to show.
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References
Adapted from (subject to copyright, with kind permission)
- Fitzpatrick, Richard: Euclid's "Elements of Geometry"