But what is the area of each of these rectangles trying? What is it an approximation for? Well, this one right over here, you have f of a, or I should say v of a. So your velocity at time a is the height right over here. And then this distance right over here is a change in time, times delta t. So the area for that rectangle is your velocity at that moment times your change in time. What is the velocity at that moment times your change in time? Well, that's going to be your change in position.