f'x(0,0)=lim(x→0)[f(x,0)-f(0,0)]/(x-0)
=lim(x→0)x^3/x
=lim(x→0)x^2
=0
由于F(x,y)={x^3-y^3/x^2+y^2在(x,y)→(0,0)的时候F(x,y)→0,所以在(0,0)的某个邻域应该是连续的,可以用定义求导