For the function f(xly ) = x2+ 4 xy- yz at the point (2, 1) (a) (6 pts) Find the direction of maximum increase of f. (Your direction should be unit vector.) (b) (2 pts) What is the rate of increase of f in this direction? (c) (2 pts) Find the direction of maximum decrease of f. (d) (4 pts) Find the directions of NO change of f. (e) (6 pts) Find the directional derivative (Dif)(2,1) where v=61,-1>