# Rectangle Contained by Medial Straight Lines Commensurable in Length is Medial

## Theorem

In the words of Euclid:

The rectangle contained by medial straight lines commensurable in length is medial.

## Proof

Let $x$ and $\lambda x$ be two medials such that:

$x \frown \lambda x$

where $\frown$ denotes that $x$ and $\lambda x$ are commensurable in length.

$x^2 : x \cdot \lambda x = x : \lambda x$
$x^2 \frown x \cdot \lambda x$

From Medial is Irrational we have that $x^2$ is medial.

$x \cdot \lambda x$ is medial.

$\blacksquare$

## Historical Note

This proof is Proposition $24$ of Book $\text{X}$ of Euclid's The Elements.