Riemann sums in summation notation

Function f of x = x cubed is graphed. The x-axis goes from negative 0.5 to 2. The graph is a curve. The curve starts in quadrant 3, moves upward through (0, 0), continues upward concave up and ends in quadrant 1. The region between the curve and the x-axis, between x = 0 and x = 2, is shaded. The shaded region is divided into 6 rectangles of width 1 third. Each rectangle touches the curve at the top left corner.