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. So we can rewrite this as our change in u approaches zero. And when we rewrite it like that, well, then this is just dy du. This is just dy, the derivative of y, with respect to u.