Rectangle areas are found by calculating height × width. the width of each rectangle equals δx and the height of each rectangle is given by the function value at the right-hand side of the rectangle. so we must calculate r4 = 4 f(xi)δx = [f(x1) + f(x2) + f(x3) + f(x4)] δx i = 1 , where x1, x2, x3, x4 represent the right-hand endpoints of four equal sub-intervals of 0, π 2 . since we wish to estimate the area over the interval 0, π 2 using 4 rectangles of equal widths, then each rectangle will have width δx = .