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. The f of x right over here is just the radius of the disk. So this part right over here is just really pi r squared. We multiply it times the depth, and then we take the sum from a to b.