Let f(x,y) = exy-y² (a) Compute the directional derivative in the direction of u = (5,12) at the point P = (1, 1). (b) Find the maximum rate of change of f at the point (1, 1). (c) Let u be a unit vector such that the angle between u and ∇f(1, 1) is radians. Find the rate of change of f in the direction of u at the point (1, 1).