evaluate the integral of the given function f (x, y) over the plane region that is described: 15. f (x, y) = xy, Ris bounded by the parabola y = x2 and the line y = 4. 16. f (x, y) = x2, Ris bounded by the parabola y = 2 - x2 and the line y = -4. 17. f (x, y) = xy, Ris bounded by the parabola y = x2 and y = 8 - x2. 18. f (x, y) = xy, Ris bounded by the parabola x = 1 - y2 and x = y2 - 1.