Proposition: Prop. 8.21: Numbers between which exist two Mean Proportionals are Similar Solid
(Proposition 21 from Book 8 of Euclid's “Elements”)
If two numbers fall between two numbers in mean proportion then the (latter) are similar solid (numbers) .
* For let the two numbers $C$ and $D$ fall between the two numbers $A$ and $B$ in mean proportion.
* I say that $A$ and $B$ are similar solid (numbers) .
Modern Formulation
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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