a的二次方+b的二次方+c的二次方-ab-bc-ca=0
2a²+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,c-a=0
a=b=c
a的二次方+b的二次方+c的二次方-ab-bc-ca=0
2a²+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,c-a=0
a=b=c