a/ ĐKXĐ: \(cosx\ne-\frac{1}{2}\)
\(\Leftrightarrow2cosx-1=6cosx+3\)
\(\Leftrightarrow4cosx=-4\Rightarrow cosx=-1\)
\(\Rightarrow x=\pi+k2\pi\)
b/
\(\Leftrightarrow cosx\left(2cos2x-1\right)-3cosx=0\)
\(\Leftrightarrow cosx\left(2cos2x-4\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}cosx=0\\cos2x=2\left(vn\right)\end{matrix}\right.\)
\(\Rightarrow x=\frac{\pi}{2}+k\pi\)
c/
\(\Leftrightarrow2sin2x.cos2x=0\)
\(\Leftrightarrow sin4x=0\)
\(\Rightarrow4x=k\pi\Rightarrow x=\frac{k\pi}{4}\)