Ta có : \(\left(a-b\right)^2\ge0\Rightarrow a^2+b^2\ge2ab\)
\(\left(b-c\right)^2\ge0\Rightarrow b^2+c^2\ge2bc\)
\(\left(a-c\right)^2\ge0\Rightarrow a^2+c^2\ge2ac\)
\(\Rightarrow2\left(a^{2+}b^2+c^2\right)\ge2ab+2bc+2ac\)
\(\Rightarrow a^2+b^2+c^2\ge ab+bc+ac\left(đpcm\right)\)
Chúc bạn học tốt !!!