# Similar Solid Numbers have Same Ratio as between Two Cubes

## Theorem

In the words of Euclid:

Similar solid numbers have to one another the ratio which a cube number has to a cube number.

## Proof

Let $a$ and $b$ be similar solid numbers.

From Between two Similar Solid Numbers exist two Mean Proportionals, there exists two mean proportionals $m_1$ and $m_2$ between them.

By definition of mean proportional:

$\left({a, m_1, m_2, b}\right)$

is a geometric sequence.

$\exists k, p, q \in Z: a = k p^3, b = k q^3, m_1 = k p^2 q, m_2 = k p q^2$

Thus:

$\dfrac a b = \dfrac {k p^3} {k q^3} = \dfrac {p^3} {q^3}$

Hence the result.

$\blacksquare$

## Historical Note

This proof is Proposition $27$ of Book $\text{VIII}$ of Euclid's The Elements.