Монгол Бодлогын Сан

Заавар:

Бодолт:

$\cos x+\sin 2x=\cos x+2\sin x\cos x=\cos x(1+2\sin x)< 0$. Иймд $$\left[\begin{array}{l}\left\{\begin{array}{c}\cos x>0\\1+2\sin x< 0\end{array}\right.\Leftrightarrow \dfrac{\pi}{2}< x< \dfrac{7\pi}{6}\\\left\{\begin{array}{c}\cos x< 0\\1+2\sin x>0\end{array}\right.\Leftrightarrow\dfrac{3\pi}{2}< x< \dfrac{11\pi}{6}\end{array}\right.$$