So it's going to be that times dx. And we want to take the sum of all of these disks from a to b. And we're going to take the sum of them and then take the limit as the dx's get smaller and smaller and smaller, and we have an infinite number of these disks. And that means we're just going to take a definite integral. So we're going to take the definite integral of this from a to b. And this right here is the formula that you will often see in a calculus book for using the disk method when you're rotating around the x-axis. But I just wanted to show you that it comes out of the common sense of finding the volume of this disk.