a) (10 pts) Let f (x, y, z) = x' + x(2+ z) y' + 2ycos x . Find all the critical points of f N and determine whether it is local minima, local maxima, saddle points or none of them. b) (8 pts) Let f(x,y) = yvx -y' -x+6y. Find all the critical points of f and determine whether it is local minima, local maxima, saddle points or none of them