x-y=m,z-y=20可得：x-z=(x-y)-(z-y)=m-20x²+y²+z²-xy-yz-zx=(2x²+2y²+2z²-2xy-2yz-2zx)/2=［(x-y)²+(z-y)²+(x-z)²］/2=［m²+20²+(m-20)²］/2=m²-...