So let's just remind ourselves a definition of a derivative. And there's multiple ways of writing this. For the sake of this video, I'll write it as the derivative of our function at point C, this is Lagrange notation with this F prime, the derivative of our function F at C, is going to be equal to the limit as X approaches C of F of X minus F of C over X minus C. And at first when you see this formula, and we've seen it before, it looks a little bit strange, but all it is is it's calculating the slope. This is our change in the value of our function, or you could think of it as our change in Y if Y is equal to F of X. And this is our change in X. And we're just trying to see, well, what is that slope as X gets closer and closer to C, as our change in X gets closer and closer to zero? And we talk about that in other videos.