Plane numbers have to one another the ratio compounded1 out of (the ratios of) their sides. * Let $A$ and $B$ be plane numbers, and let the numbers $C$, $D$ be the sides of $A$, and (the numbers) $E$, $F$ (the sides) of $B$. * I say that $A$ has to $B$ the ratio compounded out of (the ratios of) their sides.
If $A=CD$ and $B=EF$ then $\frac AB=\frac CE\cdot \frac DF.$
i.e., multiplied (translator's note) ↩