Euclid Book IX: Miscellaneous Topics from Arithmetic

The first two propositions of Book IX have already been dealt with in the section on similar plane numbers. Props. IX. 3-6 are concerned with cube numbers, for instance Prop. IX. 3. x is a cube ⇒ x2 is a cube. Prop. IX. 6. x2 is a cube ⇒ x is a cube.