第1个回答 2011-01-25
x^2+y^2+z^2-xy-yz-zx
=1/2(2(x^2+y^2+z^2-xy-yz-zx ))
=1/2(2x^2+2y^2+2z^2-2xy-2yz-2zx)
=1/2((x^2-2xy+y^2)+(x^2-2xz+z^2)+(y^2-2yz+z^2))
=1/2((x-y)^2+(x-z)^2+(y-z)^2)
x-y=m,y-z=n (x-y)+(y-z)=x-z=m+n
上式=1/2(m^2+(m+n)^2+n^2)
=1/2(m^2+m^2+2mn+n^2+n^2)
=1/2(2m^2+2mn+2n^2)
=m^2+mn+n^2