9. Dene f : R2 > R by f(:c, y) = 3531/4 + 2:833; + y. (a) (6 points) Compute the directional derivative of f at (3, 1) in the direction of (1,2). (b) (6 points) Let g(t) = f (3 + t,1 + 2t). Compute g'(0) using the chain rule. (0) (2 points) Briey explain Why the answers in part (a) and part (b) are different