In the last video, we attempted to approximate the area under a curve by constructing four rectangles of equal width and using the left boundary of each rectangle, the function evaluated at the left boundary, to determine the height. And we came up with an approximation. What I want to do in this video is generalize things a bit, using the exact same method, but doing it for an arbitrary function with arbitrary boundaries and an arbitrary number of rectangles. So let's do it. I'm going to draw the diagram as large as I can to make things clear, to make things as clear as possible. So that's my y-axis. And this right over here is my x-axis.