f(x, y) = 4et cosy, (a, b) = (0, - ), and v = (4, 3). a. Calculate the directional derivative of f at the point (a, b) in the direction defined by U. b. Find the direction at (a, b) in which the rate of change of f is greatest. c. Find the maximum rate of change. d. Fill in the blank: f decreases the most at (a, b) in the direction of