Proposition: Prop. 10.086: Construction of Second Apotome
(Proposition 86 from Book 10 of Euclid's “Elements”)
To find a second apotome.
- Let the rational (straight line) A, and GC (which is) commensurable in length with A, be laid down.
- Thus, GC is a rational (straight line).
- And let the two square numbers DE and EF be laid down, and let their difference DF be not square [Prop. 10.28 lem. I] .
- And let it have been contrived that as FD (is) to DE, so the square on CG (is) to the square on GB [Prop. 10.6 corr.] .
- Thus, the square on CG is commensurable with the square on GB [Prop. 10.6].
- And the (square) on CG (is) rational.
- Thus, the (square) on GB [is] also rational.
- Thus, BG is a rational (straight line).
- And since the square on GC does not have to the (square) on GB the ratio which (some) square number (has) to (some) square number, CG is incommensurable in length with GB [Prop. 10.9].
- And they are both rational (straight lines).
- Thus, CG and GB are rational (straight lines which are) commensurable in square only.
- Thus, BC is an apotome [Prop. 10.73].
- So, I say that it is also a second (apotome) .

Modern Formulation
This proposition proves that the second apotome has length
\frac{\alpha}{\sqrt{1-\beta^{\,2}}}-\alpha,
where \alpha,\beta denote positive rational numbers.
Notes
See also [Prop. 10.49].
Table of Contents
Proofs: 1
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Propositions: 1
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References
Adapted from (subject to copyright, with kind permission)
- Fitzpatrick, Richard: Euclid's "Elements of Geometry"
Adapted from CC BY-SA 3.0 Sources:
- Prime.mover and others: "Pr∞fWiki", https://proofwiki.org/wiki/Main_Page, 2016