Problem 3: Sketch the vector fields F(x, y) = (y,0) and G(x, y) = (0, y). Problem 4: Find the potential function for the vector field F(x, y) = (ycos (x) + 1, sin (x) + 3y?). Problem 5: Suppose that f(x, y) is any twice continuously differentiable multivariable function, and let (P(x, y). Q(r, y)) = Vf(x, y). Calculate Q, - P, (sometimes called the "2D curl" of the vector field (P,Q) ) in terms of martial derivatives of f