So this is the volume, this is the volume if I were to just rotate f of x around y is equal to one. And actually I should just write dx here. And so this right over here, this expression, I just did that, so they really are equal. 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.