I think you see where this is going. Now, this right over here, just looking at it the way it's written right here, we can't quite yet call this dy du, because this is the limit as delta x approaches zero, not the limit as delta u approaches zero. But we just have to remind ourselves the results from probably the previous video, depending on how you're watching it, which is, if we have a function u that is continuous at a point that as delta x approaches zero, delta u approaches zero. So we can actually rewrite this. We can rewrite this right over here. Instead of saying delta x approaches zero, that's just going to have the effect, because u is differentiable at x, which means it's continuous at x, that means that delta u is going to approach zero. As our change in x gets smaller and smaller and smaller, our change in u is going to get smaller and smaller and smaller.