证明：∵x²+y²+z²-xy-yz-xz=0∴2x²+2y²+2z²-2xy-2yz-2xz=0∴（x²-2xy+y²）+（x²-2xz+z²）+（z²-2yz+y²）=0∴（x-y）²+（x-z）²+（z-y）...