Các câu 1,2,4,5 làm rồi
3.
\(2cos\left(x-\pi\right)-1=0\)
\(\Leftrightarrow cos\left(x-\pi\right)=\dfrac{1}{2}\)
\(\Leftrightarrow-cosx=\dfrac{1}{2}\)
\(\Leftrightarrow cosx=-\dfrac{1}{2}\)
\(\Leftrightarrow x=\pm\dfrac{2\pi}{3}+k2\pi\)
6.
\(\Leftrightarrow\left[{}\begin{matrix}cosx=-2< -1\left(vô-nghiệm\right)\\cos2x=\dfrac{1}{3}\end{matrix}\right.\)
\(\Leftrightarrow2x=\pm arccos\left(\dfrac{1}{3}\right)+k2\pi\)
\(\Leftrightarrow x=\pm\dfrac{1}{2}arccos\left(\dfrac{1}{3}\right)+k\pi\)
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