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$cos2x-\sqrt{3}sin2x-\sqrt{3}sinx-cosx+4=0$
mình thanks trước nha@@@@@@@@@@@
$\cos 2x-\sqrt{3}\sin 2x-\sqrt{3}\sin x-\cos x+4=0 \\
\Leftrightarrow \cos 2x .\dfrac{ 1}{2} - \sin 2x \dfrac{ \sqrt{ 3}}{2} -\cos x .\dfrac{ 1}{2} - \sin x \dfrac{ \sqrt{ 3}}{2}=-2 \\
\Leftrightarrow \cos \left (2x-\dfrac{ \pi}{3} \right )-\cos \left (x-\dfrac{ \pi}{3} \right) =-2
$
Vì $\begin{cases}
\cos \left (2x-\dfrac{ \pi}{3} \right ) \ge -1 \\
-\cos \left (x-\dfrac{ \pi}{3} \right) \ge -1
\end{cases} \Rightarrow \cos \left (2x-\dfrac{ \pi}{3} \right )-\cos \left (x-\dfrac{ \pi}{3} \right) \ge -2$
Dấu bằng: $\begin{cases}
\cos \left (2x-\dfrac{ \pi}{3} \right ) = -1 \\
-\cos \left (x-\dfrac{ \pi}{3} \right) = -1
\end{cases}$
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