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. So if I were to take the function evaluated at the c, that's f of c right over here. So if I were to take the function evaluated at the c, which would essentially be the height of this line, and I multiply it times the base, this interval, if I multiply it times the interval, and this interval is just delta x, x plus delta x minus x is just delta x. So if we just multiply the height times the base, that this is going to be equal to the area under the curve, which is the definite integral from x to x plus delta x, x to x plus delta x of f of t dt.