First Apotome of Medial is Irrational

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In the words of Euclid:

If from a medial straight line there be subtracted a medial straight line commensurable with the whole in square only, and which contains with the whole a rational rectangle, the remainder is irrational. And let it be called a first apotome of a medial straight line.

(The Elements: Book $\text{X}$: Proposition $74$)



Let $AB$ be a medial straight line.

Let a medial straight line $BC$ such that:

$BC$ is commensurable in square only with $AB$
the rectangle contained by $AB$ and $BC$ is rational

be cut off from $AB$.

We have that $AB$ and $BC$ are medial.

So by definition $AB^2$ and $BC^2$ are both medial.

But $2 \cdot AB \cdot BC$ is rational.


$AB^2$ and $BC^2$ are incommensurable with $2 \cdot AB \cdot BC$.

From Proposition $16$ of Book $\text{X} $: Incommensurability of Sum of Incommensurable Magnitudes:

if $AB$ is incommensurable with either $AC$ or $CB$, $AC$ and $CB$ are incommensurable with each other.

Therefore by Proposition $7$ of Book $\text{II} $: Square of Difference:

$2 \cdot AB \cdot BC$ is incommensurable with $AC^2$.

Therefore $AC^2$ is irrational.

Therefore by definition $AC$ is irrational.

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


Historical Note

This proof is Proposition $74$ of Book $\text{X}$ of Euclid's The Elements.