But this is a super useful way of evaluating definite integrals and finding the area under a curve. Second fundamental theorem of calculus, very closely tied to the first fundamental theorem, which we won't talk about now. So why is this such a big deal? Well, let me write it in a more general notation, the way that you might be used to seeing it in your calculus book. It's telling us that if we want the area under the curve between two points, a and b, between two x points, a and b, of f of x. And so this is how we would denote the area under the curve between those two intervals. So let me draw that just to make it clear what I'm talking about in general terms.