1+1=2 em nhé
\(\begin{array}{l}
{x^2} + {y^2} + {z^2} \ge xy + yz + xz\\
\Leftrightarrow 2{x^2} + 2{y^2} + 2{z^2} - 2xy - 2yz - 2xz \ge 0\\
\Leftrightarrow \left( {{x^2} - 2xy + {y^2}} \right) + \left( {{x^2} - 2xz + {z^2}} \right) + \left( {{z^2} - 2yz + {y^2}} \right) \ge 0\\
\Leftrightarrow {\left( {x - y} \right)^2} + {\left( {x - z} \right)^2} + {\left( {z - y} \right)^2} \ge 0\,\,\,\left( {luôn\,đúng} \right)
\end{array}\)