a^2+b^2+c^2-ab-bc-ac=0.5*(2a^2+2b^2+2c^2-2ab-2bc-2ac)=(a²-2ab+b²+b²-2bc+c²+a²-2ac+b²)/2=[(a-b)²+(b-c)²+(a-c)²]/2=(a-b)²/2+(b-c)²/2+(a-c)²/2