初一数学：若x-3=y-2=z-1,求x方+y方+z方-xy-yz-zx的值 过程

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x-3=y-2=z-1=k
x=3+ky=2+kz=1+kx方+y方+z方-xy-yz-zx
=2(x方+y方+z方-xy-yz-zx)/2
=(x-y)^2+(y-z)^2+(x-z)^2/2
=(1+1+4)/2
=3