Cavalieri’s principle states that if two solids of equal height have equal cross-sectional areas at every level parallel to the respective bases, then the two solids have equal volume. the two shaded solids both have a height of 2r units. at every level, the areas of the cross sections of both solids equal π(r2 – b2). therefore, the formula for the volume of the sphere can be derived by writing an expression that represents the volume of one cone within the cylinder. the two cones within the cylinder. the solid between the two cones and the cylinder. the cylinder.