4. (18%) Given (x) = 2x- + 6, find the exact area A of the region under y = f (x) on the interval [1, 3] by first computing E f(x, )Ax, where Ax = = and then taking the limit as n 1=1 > co. (Do not use integration technique; use the computational algorithms only. You must show all the computation steps to get the credit. You may use the formulae: _ _ji = n(n+1)/2, and Et_, iz = n(n + 1)(2n + 1)/6.)