It's equal to all of this business right over here. So if you were to take all of this green area, which is from a to x plus delta x, and subtract out this blue area, which is exactly what we're doing in the numerator, what are you left with? Well, you're going to be left with what color have I not used yet? Maybe I will use this pink color. Well, no, I already used that. I'll use this purple color. You're going to be left with this area right over here.