sketch the graph of the function y=20x-x^2, and approximate the area under the curve in the interval [0, 20] by dividing the area into the given numbers of rectangles. Part A; use five rectangles to approximate the area under the curve. Part B: use 10 rectangles to approximate the area under the curve. Part C: calculate the area under the curve using rectangles as their number becomes arbitrarily large (tends to infinity). you do not need to sketch the rectangles.