x²+y²+z²-xy-yz-xz=1/2(2x²+2y²+2z²-2xy-2yz-2xz)=1/2(x²-2xy+y²+x²-2xz+z²+y²-2yz+z²)=1/2[(x-y)²+(x-z)²+(y-z)²又x-y=5 y-z=2 两式...