Proposition: Prop. 9.19: Condition for Existence of Fourth Number Proportional to Three Numbers
Euclid's Formulation
For three given numbers, to investigate when it is possible to find a fourth (number) proportional to them.
* Let $A$, $B$, $C$ be the three given numbers.
* And let it be required to investigate when it is possible to find a fourth (number) proportional to them.
Modern Formulation
(not yet contributed)
Table of Contents
Proofs: 1
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