`~rai~`
\(b)\cos(x+30^\circ)+2\cos^215^\circ=1\\\Leftrightarrow \cos(x+30^\circ)+2\cos^215^\circ-1=0\\\Leftrightarrow \cos(x+30^\circ)+\cos30^\circ=0\\\Leftrightarrow \cos(x+30^\circ)=-\cos30^\circ\\\Leftrightarrow \cos(x+30^\circ)=\cos(180^\circ-30^\circ)\\\Leftrightarrow \cos(x+30^\circ)=\cos150^\circ\\\Leftrightarrow \left[\begin{array}{I}x+30^\circ=150^\circ+k360^\circ\\x+30^\circ=-150^\circ+k360^\circ\end{array}\right.\\\Leftrightarrow \left[\begin{array}{I}x=120^\circ+k360^\circ\\x=-180^\circ+k360^\circ.\end{array}\right.\quad(k\in\mathbb{Z})\\\text{Vậy S=}\{120^\circ+k360^\circ;-180^\circ+k360^\circ|k\in\mathbb{Z}\}.\)
