And remember, also, if they're differentiable at x, that means they're continuous at x. But if u is differentiable at x, then this limit exists, and this is the derivative of, this is u prime of x, or du dx. So this right over here, we can rewrite as du dx. 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.