14x+y+z=2,分别乘以x,y,z得x方=2x-xy-xzy方=2y-xy-xzz方=2z-xz-yzx^2+y^2+z^2=2x-xy-xz+2y-xy-xz+2z-xz-yz=2(x+y+z)-2(xy+xz+yz)=2X2-2(-5)=4+10=14
因为:x+y+z=2同乘以x:x^2+xy+xz=2x同乘以y:xy+y^2+yz=2y同乘以z:xz+yz+z^2=2z3式相加:x^2+y^2+z^2+2(xy+xz+yz)=2(x+y+z)x^2+y^2+z^2=2(x+y+z)-2(xy+xz+yz)=2*2-2*(-5)=4+10=14