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. The mean value theorem of definite integrals tells us, the mean value theorem of definite integrals tells us, there exists a c in the interval. So I could see where, I'll write it this way, where a is less than or equal to c, which is less than, or actually, let me make it clear, the interval that we now care about is between x and x plus delta x. Where x is less than or equal to c, which is less than or equal to x plus delta x, such that the function evaluated at c, so let me draw this c. So there's a c someplace over here.