（2x²+2y²+2z²-2xy-2xz-2yz）/2
=【x²-2xy+y²+y²-2yz+z²+z²-2zx+x²】/2
=【（x-y）²+（y-z）²+（z-x）²】/2
x-3=y-2
x-y=1
x-3=z-1
x-z=2
y-2=z-1
y-z=1
代入