find the directional derivative using f(x, y, z) = xy + z^2.

19. At (1, 2. 3) in the direction of i + / +k. 20. At (1, 1. 1) in the direction of / + 27 + 3k .23. At the point (2, 3, 4) in the direction of a vector making an angle of 3x/4 with grad f (2, 3, 4)