Let f(x, y, z) = cos(x +2y + 3z) and (xo, Yo, Zo) = (,). a) Find the maximal directional derivative of f at (xo, Yo, Zo). b) 1 2 Calculate Duf at (xo, Yo, Zo), for u = (-3) using the definition Duf = limt-o(f((xo, Yo, Zo) + tu) f(xo, Yo, Zo)). Verify that Duf u. Vf at this point.