Part (c), (d) and (e) of the following problem:
My attempt (please correct me where I am wrong)
From part (a) I know that . Therefore:
(d) Letting we will have . 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.