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