When approximating an area with {{MATH_43}} rectangles, the {{CMD_44}} {{MATH_45}} are the {{MATH_46}}-coordinates that determine the edges of the rectangles. In the graph below, we've numbered the rectangles to help you see the relation between the indices of the grid points and the {{MATH_47}}th rectangle. {{CMD_48}} {{ENV_49}} {{CMD_50}} Note, if we are approximating the area between a curve and the horizontal axis on {{MATH_51}} with {{MATH_52}} rectangles, then it is always the case that {{MATH_53}} {{CMD_54}}