# Two Irrational Straight Lines arising from Medial Area from which Medial Area Subtracted

## Theorem

In the words of Euclid:

If from a medial area there be subtracted a medial area incommensurable with the whole, the two remaining irrational straight lines, either a second apotome of a medial straight line or a straight line which produces with a medial area a medial whole.

## Proof

Let $BC$ be a medial area.

Let the rational area $BD$ be subtracted from $BC$.

It needs to be demonstrated that the "side" of the remainder $EC$ is either:

a second apotome of a medial straight line

or:

a straight line which produces with a medial area a medial whole.

Let $FG$ be a rational straight line.

Let the rectangle $GH$ be applied to $FG$ equal to $BC$ producing $FH$ as breadth.

Let the area $GK$ equal to $BC$ be subtracted from $GH$.

Then the remainder $EC$ is equal to $LH$.

We have that:

$BC$ and $BD$ are medial.

and:

$BC$ is incommensurable with $BD$.

We also have:

$BC = GH$

and:

$BD = GK$

Therefore $GH$ and $GK$ are medial.

$FH$ and $FK$ are rational and incommensurable in length with $FG$.

We have that $BC$ is incommensurable with $BD$.

Therefore $GH$ is incommensurable with $GK$.

From:

Proposition $1$ of Book $\text{VI}$: Areas of Triangles and Parallelograms Proportional to Base

and:

Proposition $11$ of Book $\text{X}$: Commensurability of Elements of Proportional Magnitudes

we have that:

$HF$ is incommensurable in length with $FK$.

Therefore $FH$ and $FK$ are rational straight lines which are commensurable in square only.

Therefore $KH$ is an apotome and $KF$ is the annex to $KH$.

We have that:

$HF^2 = FK^2 + \lambda^2$

where either:

$\lambda$ is commensurable in length with $HF$

or:

$\lambda$ is incommensurable in length with $HF$.

First suppose $\lambda$ is commensurable in length with $HF$.

We have that neither $FH$ and $FK$ is commensurable in length with the rational straight line $FG$.

Therefore $KH$ is a third apotome.

the "side" of $LH$ is a second apotome of a medial straight line.

Next suppose $\lambda$ is incommensurable in length with $HF$.

We have that neither $FH$ and $FK$ is commensurable in length with the rational straight line $FG$.

Therefore $KH$ is a sixth apotome.

the "side" of $LH$ is a straight line which produces with a medial area a medial whole.

$\blacksquare$

## Historical Note

This proof is Proposition $110$ of Book $\text{X}$ of Euclid's The Elements.