That's the whole power of the definite integral. And so you can imagine as these dx's get smaller and smaller and smaller, these rectangles get narrower and narrower and narrower, and we have more of them, we're getting a better and better approximation of the area under the curve until at the limit we are getting the area under the curve. Now we're going to apply that same idea, not to find the area under this curve, but to find the volume if we were to rotate this curve around the x axis. So this is going to stretch our powers of visualization here. So let's think about what happens when we rotate this thing around the x axis. So let's figure to rotate it, and I'll look at, say we're looking at a little bit from the right. So we get kind of a base that looks something like this.