z=F(x^2+y^2+z^2)-x-y.
dz/dx=2xF'+2z*(dz/dx)F'-1,
dz/dy=2yF'+2z*(dz/dy)F'-1,
-> (y-z)*dz/dx+(z-x)*dz/dy=(y-x)(2zF'-1)+2zF'[(y-z)*dz/dx+(z-x)*dz/dy]
-> (y-z)*dz/dx+(z-x)*dz/dy=x-y (2zF'-1!=0).
z=F(x^2+y^2+z^2)-x-y.
dz/dx=2xF'+2z*(dz/dx)F'-1,
dz/dy=2yF'+2z*(dz/dy)F'-1,
-> (y-z)*dz/dx+(z-x)*dz/dy=(y-x)(2zF'-1)+2zF'[(y-z)*dz/dx+(z-x)*dz/dy]
-> (y-z)*dz/dx+(z-x)*dz/dy=x-y (2zF'-1!=0).