Part(c),(d)and(e)of the following problem:

http://img368.imageshack.us/img368/568/61463247.gif

My attempt (please correct me where I am wrong)

(c)so

From part (a) I know that . Therefore:

Therefore

(d)Letting we will have . Therefore

Therefore

I got 0 again!

(e)I think maybe the method in part (d) is more efficient but neither work here because in part (b) I showed that the function is not differentiable at (0,0), and if a function is not differentiable at a point then the directional derivative does not exist there for all directions .

Is this right? I need to submit this tomorrow so I would appreciate any help or suggestions.