Now, what does this represent? Remember, we don't know anything about definite integrals or somehow dealing with something with an antiderivative and all that. We just know this is another way of saying the area under the curve f between a and x plus delta x. So it's the area under the curve f between a and x plus delta x. So it's this entire area right over here. So that's this part. We already know what this blue stuff is.