1.
f(x,y) = 0 , if (x,y)=(0,0)
x^3
--------------- , other
(x^2+y^2)^(1/2)
求: f在x in R 可全微分
2.
f(x,y) = 1, if exist t in R, t /= 0 with (x,y) = (t,t^2)
0, other
求: f在(0,0)不可全微分 但Directional derivative皆存在
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