Let f(x) - 4x. (a) Sketch the region R under the graph of fon the interval (0,2], Find its exact area using geometry. square units (b) Use a Riemann sum with four subintervais of equal length (n 4) to approximate the area of R. Choose the representative points to be the left endpaints of the subintervals square units (c) Repeat part (b) with eight subintervals of equal length (n B). square units ) Compara the nrimarinn ohtaine