6. (a) Find the directional derivative of f(x, y, z) = ex -arts at Po(1, 1, 0) in the direction of u = i - 2j + k. [4 marks] (b) Let z = 1201 2018 / ), where f is a differentiable function, show that I + 2y dy 2018z. [4 marks] (c) Use Lagrange Multiplier to find the absolute maximum and minimum of f(x, y) = x' + y' subject to r' + 2y' = 10 [12 marks]