a的二次方+b的二次方+c的二次方-ab-bc-ca=02a²+2b²+2c²-2ab-2bc-2ca=0(a²-2ab+b²)+(b²-2bc+c²)+(c²-2ca+a²)=0(a-b)²+(b-c)²+（c-a)²=0所以a-b=0,b-c=0...