X^2+Y^2+Z^2=XY+YZ+XZ
则有2X^2+2Y^2+2Z^2-2XY-2YZ-2XZ=0
==>(X-Y)^2+(Y-Z)^2+(Z-X)^2=0
必然 X-Y=0,Y-Z=0,Z-X=0
==>X=Y=Z