\[\begin{array}{l}
\cos 5x + {m^2} - 4 = 0 \Leftrightarrow \cos 5x = 4 - {m^2}\\
PT\,vo\,nghiem\, \Leftrightarrow \left[ \begin{array}{l}
4 - {m^2} > 1\\
4 - {m^2} < - 1
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
{m^2} < 3\\
{m^2} > 5
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
- \sqrt 3 < m < \sqrt 3 \\
\left[ \begin{array}{l}
m > \sqrt 5 \\
m < - \sqrt 5
\end{array} \right.
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
m > \sqrt 5 \\
m < - \sqrt 5 \\
- \sqrt 3 < m < \sqrt 3
\end{array} \right.
\end{array}\]
