由X-Y=5(根号2) Z-Y=10 得Z-X=10-5√2 X2+Y2+z2-XY-YZ-XZ=1/2（2X2+2Y2+2z2-2XY-2YZ-2XZ）=1/2[(X-Y)^2+(Z-Y)^2+(Z-X)^2] =150-50√2