Side of Area Contained by Rational Straight Line and Fifth Apotome
Theorem
In the words of Euclid:
- If an area be contained by a rational straight line and a fifth apotome, the "side" of the area is a straight line which produces with a rational area a medial whole.
(The Elements: Book $\text{X}$: Proposition $95$)
Proof
Let the area $AB$ be contained by the rational straight line $AC$ and the fifth apotome $AD$.
It is to be proved that the "side" of $AB$ is a straight line which produces with a rational area a medial whole.
Let $DG$ be the annex of the fifth apotome $AD$.
Then, by definition:
- $AG$ and $GD$ are rational straight lines which are commensurable in square only
- the annex $GD$ is commensurable with the rational straight line $AC$
- the square on the whole $AG$ is greater than the square on the annex $GD$ by the square on a straight line which is incommensurable in length with $AG$.
Let there be applied to $AG$ a parallelogram equal to the fourth part of the square on $GD$ and deficient by a square figure.
- that parallelogram divides $AG$ into incommensurable parts.
Let $DG$ be bisected at $E$.
Let the rectangle contained by $AF$ and $FG$ be applied to $AG$ which is equal to the square on $EG$ and deficient by a square figure.
Therefore $AF$ is incommensurable with $FG$.
We have that $AG$ commensurable in length with $AC$, and both are rational.
Therefore from Proposition $21$ of Book $\text{X} $: Medial is Irrational:
We have that $DG$ isincommensurable in length with $AC$, while both are rational.
Therefore from Proposition $21$ of Book $\text{X} $: Medial is Irrational:
- the rectangles $DK$ is medial.
Let the square $LM$ be constructed equal to $AI$.
Let the square $NO$ be subtracted from $LM$ having the common angle $\angle LPM$ equal to $FK$.
Therefore from Proposition $26$ of Book $\text{VI} $: Parallelogram Similar and in Same Angle has Same Diameter:
Let $PR$ be the diameter of $LM$ and $NO$.
It is to be shown that $LN$ is the "side" of the area $AB$.
We have that $AK$ is medial and equals $LP^2 + PN^2$.
Therefore $LP^2 + PN^2$ is medial.
We have that the rectangle $DK$ is rational.
But $DK$ equals twice the rectangle contained by $LP$ and $PN$.
Therefore twice the rectangle contained by $LP$ and $PN$ is rational.
We have that $AI$ is incommensurable with $FK$.
Therefore $LP^2$ is incommensurable with $PN^2$.
Therefore $LP$ and $PN$ are straight lines which are incommensurable in square such that $LP^2 + PN^2$ is medial and such that $2 \cdot LP \cdot PN$ is rational.
Therefore by definition $LN$ is a straight line which produces with a rational area a medial whole.
But $LN$ is the "side" of the area $AB$.
Hence the result.
$\blacksquare$
Historical Note
This proof is Proposition $95$ of Book $\text{X}$ of Euclid's The Elements.
It is the converse of Proposition $101$: Square on Straight Line which produces Medial Whole with Rational Area applied to Rational Straight Line.
Sources
- 1926: Sir Thomas L. Heath: Euclid: The Thirteen Books of The Elements: Volume 3 (2nd ed.) ... (previous) ... (next): Book $\text{X}$. Propositions