Let's say this right over here, that is c. This right over here is u of c. u of c. And then let's just take some arbitrary x over here. So some arbitrary x, and then this right over here, this right over here is u of x. is u of x. So if we define, if we define our change, if we define, let me do this. 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.