a²+b²+c²-ab-bc-ac=1/2*(2a²+2b²+2c²-2ab-2bc-2ac)=1/2*[(a²-2ab+b²)+(b²-2bc+c²)+(c²-2ac+a²)]=1/2*[(a-b)²+(b-c)²+(c-a)²]
