You take the derivative of the indefinite integral where the upper boundary is x right over here, it just becomes whatever you were taking the integral of, that as a function instead of t, that is now a function of x. So it can really simplify sometimes taking a derivative. And sometimes you'll see on exams these trick problems where you have this really hairy thing that you take a definite integral of and then take the derivative. And you just have to remember the fundamental theorem of calculus, the thing that ties it all together, connects derivatives and integration, that you can just simplify it by realizing that this is just going to be, instead of a function of lowercase f of t, it's going to be lowercase f of x. Let me make it clear. In this example right over here, this right over here was lowercase f of t, and now it became lowercase f of x. This right over here was our a.