x-y=a
z-y=6
所以x-z=(x-y)+(y-z)=a-6
x^2+y^2+z^2-xy-yz-xz=[(x-y)^2+(y-z)^2+(z-x)^2]/2
=[a^2+36+(a-6)^2]/2
=a^2-6a+36
=(a-3)^2+27>=27
最小值27,当a=3取到