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. So I'm now going to make a few claims in this video. And I'm not going to prove them rigorously. There is another video that will go a little bit more into the proof direction.