Straight Line Commensurable with Apotome of Medial Straight Line
Theorem
In the words of Euclid:
- A straight line commensurable with an apotome of a medial straight line is an apotome of a medial straight line and the same in order.
(The Elements: Book $\text{X}$: Proposition $104$)
Proof
Let $AB$ be an apotome of a medial straight line.
Let $CD$ be commensurable in length with $AB$.
It is to be demonstrated that:
- $CD$ is an apotome of a medial straight line
and:
Let $BE$ be the annex of $CD$.
Therefore by definition of apotome of a medial straight line:
- $AE$ and $EB$ are medial straight lines which are commensurable in square only.
From Proposition $12$ of Book $\text{VI} $: Construction of Fourth Proportional Straight Line, let it be contrived that:
- $BE : DF = AB : CD$
From Proposition $12$ of Book $\text{V} $: Sum of Components of Equal Ratios:
- $AE : CF = AB : CD$
and so from Proposition $11$ of Book $\text{V} $: Equality of Ratios is Transitive:
- $AE : CF = BE : DF$
But $AB$ is commensurable in length with $CD$.
Therefore from Proposition $11$ of Book $\text{X} $: Commensurability of Elements of Proportional Magnitudes:
- $AE$ is commensurable in length with $CF$
and:
- $BE$ is commensurable in length with $DF$.
We have that $AE$ and $EB$ are medial straight lines which are commensurable in square only.
- $CF$ and $FD$ are medial straight lines.
Therefore from Proposition $13$ of Book $\text{X} $: Commensurable Magnitudes are Incommensurable with Same Magnitude:
- $CF$ and $FD$ are medial straight lines which are commensurable in square only.
Thus $CD$ is an apotome of a medial straight line.
$\Box$
It remains to be shown that the order of $CD$ is the same as the order of $AB$.
We have that:
- $AE : CF = BE : DF$
So from Proposition $16$ of Book $\text{V} $: Proportional Magnitudes are Proportional Alternately:
- $AE : EB = CF : FD$
Therefore:
- $AE^2 : AE \cdot EB = CF^2 : CF \cdot FD$
But $AE^2$ is commensurable with $CF^2$.
Therefore from:
and:
we have that:
- $AE \cdot EB$ is commensurable with $CF \cdot FD$.
Therefore by Book $\text{X}$ Definition $4$: Rational Area:
Thuis $CD$ is an apotome of a medial straight line of the same as the order as $AB$.
$\blacksquare$
Historical Note
This proof is Proposition $104$ of Book $\text{X}$ of Euclid's The Elements.
Sources
- 1926: Sir Thomas L. Heath: Euclid: The Thirteen Books of The Elements: Volume 3 (2nd ed.) ... (previous) ... (next): Book $\text{X}$. Propositions