4. [10p] Consider a parametric curve F (t ) = (x(t), (t) ), I(t) = 2 cos(t), u(t) = sin(t) (i) [7p) Find the critical points of f(x(t), y(t)), where f(x, y) = Vx2 + 12 is the dis- tance function between (r, y) and (0, 0).