Use Green's theorem to evaluate the line integral along the given positively oriented, simple closed curve.
(a) ∮ ye^x dx + 2e^y dy where C is the rectangle with vertices (0,0), (3,0), (3,4), (0,4).
(b) ∮ (x^2 + y^2) dx + (x^2 - y^2) dy where C is the triangle with vertices (0,0), (2,1), and (0,1).
(c) ∮ y^3 dx - x^3 dy where C is the circle x^2 + y^2 = 4.
(d) ∮ (y + e^v) dx + (2x + cos(y^2)) dy where C is the boundary of the region enclosed by the parabolas y = x^2 and x = y^2.