This is obviously just the volume of each of those disks. So this is the volume if I were to take the f of x around y equals one. Let's figure out, so let me call this volume of f of x. And by the same logic, the same exact logic, we can figure out the volume if we take g of x, if we rotate g of x around, if we rotate g of x, if we construct disks like this and rotate them around y is equal to one. And so the volume, if I take g of x around y equals one, would be zero to 1 1⁄2 times pi times one minus g of x, because one minus g of x is each of these radiuses right over here, each of these radiuses. That squared dx. And so the volume of what they're asking us, the volume of the solid generated when r is rotated, well, r is kind of the space in between f of x and g of x.