"x²+y²+z²+xy+yz+xz"应该是"x²+y²+z²-xy-yz-xz"吧x-y=3/2,z-y=5/2两式相减得x-z=-1x²+y²+z²-xy-yz-xz =1/2(2x²+2y²+2z²-2xy-2yz-2xz)=1/2[(x²-2x...确定吗