Definition: 5.13: Inverse Ratio
An inverse ratio is a taking of the (ratio of the) following (magnitude) as the leading and the leading (magnitude) as the following.
Modern Formulation
In other words, given a ratio of positive real numbers \(\alpha,\beta\):
\[\frac\alpha\beta,\]
the inverse ratio (or reciprocal ratio) is given by
\[\frac\beta\alpha.\]
Mentioned in:
Proofs: 1 2 3 4 5 6 7 8
Propositions: 9 10 11 12 13
Thank you to the contributors under CC BYSA 4.0!
 Github:

 nonGithub:
 @Fitzpatrick
References
Bibliography
 Health, T.L.: "The Thirteen Books of Euclid's Elements  With Introduction and Commentary by T. L. Health", Cambridge at the University Press, 1968, Vol 1, 2, 3
Adapted from (subject to copyright, with kind permission)
 Fitzpatrick, Richard: Euclid's "Elements of Geometry"