This I can write as 1 over delta x times the numerator. We already figured out the numerator. The green area minus the blue area is just the purple area. And another way of denoting that area is this expression right over here. So 1 over delta x times the definite integral from x to x plus delta x of f of t dt. Now, this expression is interesting. This might look familiar from the mean value theorem of definite integrals.