x =1+yz =-1+yx2+y2+z2-xy-yz-xz的值x2+y2+z2-xy-yz-xz=(1+y)2 +y2+(-1+y)2-(1+y)y-y(-1+y)-( 1+y)( -1+y)=1+ y2+2y+y2+1+ y2-2y-1y- y2+1y- y2+1-1y+1y- y2=3
