If we define our change in, our change in u is equal to u of x minus u of c, which makes sense, because this is our change in u. So let's say this is going to be u of x minus u of c. And if we define our change in x is equal to x minus c, which it is in this case, it is x minus c. It is x minus c. Then we can rewrite this limit right over here. Instead of saying the limit as x approaches c, we could write the limit as delta x approaches zero, because if x approaches c, then delta x is going to approach zero. So we could write this, the limit as delta x approaches zero of delta u, of delta u, is going to be equal to zero. This, we define this as our change in u, and it is our change in u. So this is equal to zero. So another way of thinking about this is as delta x approaches zero, our change in u, our change in the function, is going to approach zero.