a²+b²+c²=ab+bc+ca
2a²+2b²+2c²=2ab+2bc+2ca
(a-b)²+(a-c)²+(c-b)²=0
a=b=c
(a+b-c)²º¹¹=(a+a-a)²º¹¹=a²º¹¹=1