But what's this going to be equal to? What's this going to be equal to? Well, the limit of the product is the same thing as the product of the limits. So this is going to be the same thing as the limit as delta x approaches zero of, and I'll color-code it, of this stuff, of delta y over delta u times, maybe I'll put parentheses around it, times, times the limit, the limit as delta x approaches zero, delta x approaches zero of this business. So let me put some parentheses around it. Delta u over delta x. So what does this simplify to?