Second Apotome of Medial is Irrational

Proof

 * Euclid-X-75.png

Let $AB$ be a medial straight line.

Let a medial straight line $CB$ such that:
 * $CB$ is commensurable in square only with $AB$
 * the rectangle contained by $AB$ and $BC$ is medial

be cut off from $AB$.

Let $DI$ be a rational straight line.

Using :
 * Let $DE$ be a parallelogram set out on $DI$ equal to $AB^2 + BC^2$.

Let its breadth be $DG$.

Similarly:
 * Let $DH$ be a parallelogram set out on $DI$ equal to $2 \cdot AB \cdot BC$.

From :
 * $FE = AC^2$

We have that $AB^2$ and $BC^2$ are medial areas which are commensurable.

Therefore from:

and:

it follows that:
 * $DE$ is medial.

We have that $DE$ has been applied to the rational straight line $DI$ producing $DG$ as breadth.

So from :
 * $DG$ is rational and incommensurable in length with $DI$.

We have that $AB \cdot BC$ is medial.

So from:
 * $2 \cdot AB \cdot BC$ is medial.

But $2 \cdot AB \cdot BC = DH$.

Therefore $DH$ is medial.

We have that $DH$ has been applied to the rational straight line $DI$ producing $DF$ as breadth.

So from :
 * $DF$ is rational and incommensurable in length with $DI$.

We have that $AB$ and $BC$ are commensurable in square only.

Therefore from :
 * $AB$ is incommensurable in length with $BC$.

But from :
 * $AB^2 + BC^2$ are commensurable with $AB^2$

and from :
 * $2 \cdot AB \cdot BC$ is commensurable with $AB \cdot BC$

therefore from :
 * $2 \cdot AB \cdot BC$ is incommensurable with $AB^2 + BC^2$.

But:
 * $DE = AB^2 + BC^2$

and:
 * $DH = 2 \cdot AB \cdot BC$

and so $DE$ is incommensurable with $DH$.

But from :
 * $DE : DH = GD : DF$

Therefore from :
 * $GD$ is incommensurable with $DF$.

But both $GD$ and $DF$ are rational.

Therefore $GD$ and $DF$ are rational straight lines which are commensurable in square only.

Therefore, by definition, $FG$ is an apotome.

But $DI$ is rational.

From :
 * a rectangle contained by a rational and an irrational straight line is irrational.

Hence its "side" is irrational.

But $AC$ is the "side" of $FE$.

Therefore $AC$ is irrational.

Such a straight line is known as a second apotome of a medial.