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? Well, this right over here, this is the definition, and we're assuming, in order for this to even be true, we have to assume that u and y are differentiable at x. So we assume, in order for this to be true, we're assuming, we're assuming y, comma, u are differentiable, are differentiable, are differentiable at x. And remember, also, if they're differentiable at x, that means they're continuous at x.