因为 xyz=1,所以 x/(xy+x+1)+y/(yz+y+1)+z/(zx+z+1) =xz/(xyz+xz+z)+y/(yz+y+xyz)+z/(zx+z+1) =xz/(1+xz+z)+1/(z+1+xz)+z/(xz+z+1) =(xz+1+z)/(1+xz+z)=1.
