f(x) = 8 + 2 / 31 + (1 / 124 * (x - 12)^(3))
n = 4 (slider from 4 to 16, step 1)
l1 = Sequence((2 + (s * 8 / n), f(2 + (s * 8 / n))), s, 0, n)
l2 = Sequence(Segment[Element[l1, i], (x(Element[l1, i]), 0)], i, 1, n + 1)
a = RectangleSum(f, 2, 10, n, 1 / 2)
g(x) = If[2 ≤ x ≤ 10, f]
Vieleck1, h, i, j, k, l = Polygon((0, 0), (4 / n, 0), (4 / n, 8), (0, 8), (0, 0))
l3 = Sequence(IntegralBetween[f(x + 2 + ((i - 1) * 8 / n)), (2 * f(2 + ((i - 1 / 2) * 8 / n))) - f((-x) + 2 + (i * 8 / n)), 0, 4 / n], i, 1, n)