Có : xyz = 1
 => x = 1/yz
      y= 1/xz
      z= 1/xy
Lại có: x+y+z = 1/x +1/y +1/z
hay    : 1/yz + 1/xz +1/xy = 1/x +1/y +1/z
  =>   1/xy + 1/xz +1/xy - 1/x -1/y -1/z = 0
<=> x/xyz + y/xyz + z/xyz - yz/xyz - xz/xyz - xy/xyz = 0
       ( x+y+z-yz-xz-xy)/xyz = 0
        (x-xy)+(y-yz)+(z-xz)/1=0
        x(1-y)+y(1-z)+z(1-x)=0