申論題

5. Let  and the surface S be the graph of the function z=f(x,y). Let P=(1,0,0)∈ S and p=(1,0) in the xy-plane.

(a) (10%) If the unit vector u (in the xy-plane) at p is the direction (among all directions at p) along which the height (i.e. the value of z) of S increases most rapidly, then u =_____