This is the area of the face of each of those disks. And then I multiply it times the depth of each of those disks. Now this is the volume of each of those disks, and I'm taking the sum of all of them. 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.