Well, that's where the second fundamental theorem of calculus is useful. I'll write it right over here. Second fundamental, I'll abbreviate a little bit, theorem, theorem of calculus. It tells us, let's say we have some function capital F of x, and it's equal to the definite integral from a, some constant a to x of lowercase f of t dt. The second fundamental theorem of calculus tells us that if our lowercase f, if lowercase f is continuous on the interval from a to x, so I'll write it this way, on the closed interval from a to x, then the derivative of our capital F of x, so capital F prime of x, is just going to be equal to our inner function f evaluated at x instead of t. It's going to become lowercase f of x. Now I know when you first saw this, you thought that, hey, this might be some cryptic thing that you might not use too often. But we're going to see that it's actually very, very useful.