令F=xyz-arctan(x+y+z)
则:Fy=xz-1/[1+(x+y+z)^2]
Fz=xy-1/[1+(x+y+z)^2]
在点(0,1,-1)处Fy=-1/5,Fz=-1/5
所以原式=-Fy/Fz=-1