Proposition: 5.13: Relative Sizes of Proportional Magnitudes
(Proposition 13 from Book 5 of Euclid's “Elements”)
If a first (magnitude) has the same ratio to a second that a third (has) to a fourth, and the third (magnitude) has a greater ratio to the fourth than a fifth (has) to a sixth, then the first (magnitude) will also have a greater ratio to the second than the fifth (has) to the sixth.
 For let a first (magnitude) $A$ have the same ratio to a second $B$ that a third $C$ (has) to a fourth $D$, and let the third (magnitude) $C$ have a greater ratio to the fourth $D$ than a fifth $E$ (has) to a sixth $F$.
 I say that the first (magnitude) $A$ will also have a greater ratio to the second $B$ than the fifth $E$ (has) to the sixth $F$.
Modern Formulation
In modern notation, this proposition reads that if \[\frac\alpha\beta=\frac\gamma\delta\text{ and }\frac\gamma\delta > \frac\epsilon\zeta\] then \[\frac\alpha\beta > \frac\epsilon\zeta\]
for all positive real numbers \(\alpha,\beta,\gamma,\delta,\epsilon,\zeta\).
Generalized Modern Formulation
see rules of calculation with inequalities (Rule 11)
Table of Contents
Proofs: 1
Mentioned in:
Proofs: 1 2 3
Sections: 4
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