a²+b²+c²=ab+bc+caa²+b²+c²-ab-bc-ac=02a²+2b²+2c²-2ab-2bc-2ac=0(a²-2ab+b²)+(b²-2bc+c²)+(c²-2ac+a²)=0(a-b)²+(b-c)²+(c-...