Then try again! and its derivative, at x= 0, is NOT 0. You seem to be under the impression that if the numerator of a fraction is 0, then the fraction is equal to 0. That's not true if thedenominatoris also 0!

No, if a function is not differentiable at a point then the directional derivativeI 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.maynot exist for some or all directions.