But we're going to see that it's actually very, very useful. And even in the future, and some of you might already know, there's multiple ways to try to think about a definite integral like this, and you'll learn it in the future. But this can be extremely simplifying, especially if you have a hairy definite integral like this. And so this just tells us, hey, look, the derivative with respect to x of all of this business, first we have to check that our inner function, which would be analogous to our lowercase f here, is this continuous on the interval from 19 to x? Well, no matter what x is, this is going to be continuous over that interval because this is continuous for all x's. And so we meet this first condition, or our major condition. And so then we could just say, all right, then the derivative of all of this is just going to be this inner function replacing t with x.