`~rai~`
\(cos(5x-45^\circ)=\dfrac{\sqrt{3}}{2}\\\Leftrightarrow cos(5x-45^\circ)=cos(30^\circ)\\\Leftrightarrow \left[\begin{array}{I}5x-45^\circ=30^\circ+k360^\circ\\5x-45^\circ=-30^\circ+k360^\circ\end{array}\right.\\\Leftrightarrow \left[\begin{array}{I}5x=75^\circ+k360^\circ\\5x=15^\circ+k360^\circ\end{array}\right.\\\Leftrightarrow \left[\begin{array}{I}x=15^\circ+k72^\circ\\x=5^\circ+k72^\circ.\end{array}\right.\\\text{Nghiệm âm lớn nhất của PT:}x=15^\circ+(-1).72^\circ=-57^\circ.\)
