Use the definition of directional derivative to solve Duf(ro, yo, 2o) at point P(xo, Yozo) where 1. 2. 3. 4. f(x, y, z) = x + y + z and u = (1, 0, 1) at P(1, 1, 1). f(x, y, z) = x + y + 2xy + z and u = (1, 0, 1) at P(1, 1, 1). f(x, y, z) = x + y + z + 2xz and u = (1, 0, 1) at P(1, 1, 1). f(x, y, z) = y + z and u = (1, 0, 1) at P(1,1,1).