So another way of thinking about this is as delta x approaches zero, our change in u, our change in the function, is going to approach zero. So as delta x approaches zero, as delta x approaches zero, delta u approaches zero. And that's what we wrote over here. Delta u approaches zero as delta x approaches zero. And in a lot of ways, this is hopefully common sense. We're dealing with a continuous function. As you get smaller and smaller, and you could just think of it this way, as you get smaller and smaller changes in x's, as our change in x gets smaller, and smaller, and smaller, and smaller, because it's continuous, and you wouldn't be able to say this for a discontinuous function, but because it's continuous, or you wouldn't be able to say this for some discontinuous functions.