Răspuns :

[tex]a^2+b^2+c^2=ab+bc+ac| \cdot 2\\ 2a^2+2b^2+2c^2=2ab+2bc+2ac\\ a^2-2ab+b^2+b^2-2bc+c^2+c^2-2ac+a^2=0\\ (a-b)^2+(b-c)^2+(c-a)^2=0=>\\ a-b=0,b-c=0,c-a=0=>\\ a=b=c[/tex]