Similar polygons inscribed in circles are to one another as the squares on their diameters.
Proposition 2 . Circles are to one another as the squares on their diameters. Lemma for XII.2. Proposition 3 . Any pyramid with a triangular base is divided into two pyramids equal and similar to one another, similar to the whole, and having triangular bases, and into two equal prisms, and the two prisms are greater than half of the whole pyramid.