\[\begin{array}{l}
{x^2} + 4x + 12 = 0\\
\Leftrightarrow {x^2} + 4x + 4 + 8 = 0\\
\Leftrightarrow {\left( {x + 2} \right)^2} + 8 = 0\\
Vi\,\,\,{\left( {x + 2} \right)^2} \ge 0\,\,\,\forall x\\
\Rightarrow {\left( {x + 2} \right)^2} + 8 > 0\,\,\,\forall x\\
\Rightarrow pt\,\,da\,\,cho\,\,vo\,\,nghiem.
\end{array}\]
