6 [5 pts] Let y = f(r) be the function graphed below on the interval [0, 9]. X A b C d B CI D The areas of the four regions determined by the vertical lines r = a, r = d, and the r-intercepts b, 0, and c, lie either above or below the x-axis and are labeled as in the figure. Their values are A = 2, B = 7, C = 13 and D = 3. a) Express the integral fo f(x) dx in terms of A, B, C, and D. Calculate its value. b) Express the area of the region enclosed by the graph of f and the r-axis when a