a^2+b^2+c^2-ab-ac-bc=1/2[(a-b)²+(b-c)²+(c-a)²]=1/2(1+1+4)=3
a^2+b^2+c^2-ab-ac-bc=1/2[(a-b)^2+(b-c)^2+(a-c)^2]=1/2(1+1+4)=3
