(a+b+c)²=3(a²+b²+c²)a²+b²+c²+2ab+2bc+2ca=3a²+3b²+3c²a²+b²+c²-ab-bc-ca=02a²+2b²+2c²-2ab-2bc-2ca=0(a-b)²+(b-c)²+...