And so let's think about those disks. So let's construct a disk on the inside right over here. So if I construct a disk on the inside, so now I'm carving out the cone part of it. What is the area of the face of one of those disks? Well, it's going to be pi times the radius squared. In this case, the radius is going to be equal to the value of this inner function, which is just x. So pi times, and so this is just y is equal to x.