3. A CAT scan of a football-shaped tumor shows slices that are spaced 2.5 centimeters apart. The areas of the slices are 0, 20, 24, 38, 56, 40, 30, 23, and 0 square centimeters. a) Write down a Riemann sum that approximates the volume of the tu- mor. b) Evaluate that Riemann sum.