Roberval showed that the area of the region between the cycloid and its companion
curve is half the area of the generating circle, or 1
2πa2. This follows from the twodimensional
version of Cavalieri’s principle: “If two regions bounded by parallel lines
are such that any parallel between them cuts each region in segments of equal length,
then the regions have equal area.” Since the companion curve is constructed so that
slices through both regions at equal heights have the same length, the areas are equal.
Roberval used Cavalieri’s principle again to show that the companion curve divides the