Răspuns :
[tex]\displaystyle\\x^2\geq 0,~(\forall)x\in\mathbb{R},~cu~egalitate~cand~x=0.\\---------------------------------\\Egalitatea~(a-b)^2+(b-c)^2+(c-a)^2=0~are~loc~daca~si~numai~daca\\a-b=b-c=c-a=0,~de~unde~se~obtine~a=b=c~si~rezulta~cerinta.[/tex]