В благодарную память о [b]uStas'е[/b][br][url]http://tube.geogebra.org/ustas[/url][br][br]Illustration of the volume of the ball using the principle of Cavalieri.[br][br]Suppose two regions in three-space (solids) are included between two parallel planes. [br]If every plane parallel to these two planes intersects both regions in cross-sections of equal area, [br]then the two regions have equal volumes.