Given two circles about the same center, to inscribe in the greater circle an equilateral polygon with an even number of sides which does not touch the lesser circle. Proposition 17 . Given two spheres about the same center, to inscribe in the greater sphere a polyhedral solid which does not touch the lesser sphere at its surface. Corollary to XII.17. Proposition 18 .
Spheres are to one another in triplicate ratio of their respective diameters.