So we could write this, the limit as delta x approaches zero of delta u, of delta u, is going to be equal to zero. This, we define this as our change in u, and it is our change in u. So this is equal to zero. 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.