And the height of this rectangle is the function evaluated at an x that's within this interval right over here. And so this part right over here is the area of one of those rectangles and we're summing them all up. And this is kind of an elongated s reminiscent of a sigma for summing. We're summing up the infinite number of those infinitely thin rectangles or the areas of those infinitely thin rectangles between 1 and 4. So that's where the notation of the definite integral comes from. But we still haven't done anything. We've just written some notation that says the exact area between 1 and 4 under the curve f of x and above the x-axis.