11. Use the change of variables formula for double integrals to calculate the following double integrals over the given region D of the plane:
(a) ∬ x^2 dxdy D = quarter disk bounded by the positive x axis, the positive y axis and the circle x^2 + y^2 = 9 (you may assume that cos^2 θ = 1/2(1 + cos 2θ);
(b) ∬ x / (x^2 + y^2)^{3/2} dxdy D = portion of annulus bounded by the circles x^2 + y^2 = 1 and x^2 + y^2 = 4 in the first quadrant x ≥ 0, y ≥ 0;
(c) ∬ y dxdy D = region bounded by the x axis and the portion of the ellipse x^2/4 + y^2/9 = 1 lying in the upper half plane, y ≥ 0.
(Note: For regions involving an ellipse whose equation is x^2/a^2 + y^2/b^2 = 1, an appropriate change of variables is x = ar cos θ y = br sin θ, 0 ≤ r < ∞, 0 ≤ θ ≤ 2π).