Tìm m để pt cos 5x +m2-4= 0 vô nghiệm

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

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