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