Đáp án+Giải thích các bước giải:
`Cos2x-cosx=2sin^2 ``(3x)/2`
`->Cos2x-cosx=1-cos3x`
`->2Cos^2x-1-cosx=1-(4cos^3x-3cosx)`
`->4cos^3x+2cos^2x-4cosx-2=0`
`->`\(\left[ \begin{array}{l}\cos x=1\\\cos x=-1\\\cos x=-\dfrac12\end{array} \right.\)
`->`\(\left[ \begin{array}{l}x=k2\pi\\x=\pi+k2\pi\\\left[\begin{matrix} \dfrac{2\pi}3+k2\pi\\-\dfrac{2\pi}3+k2\pi \end{matrix}\right.\end{array} \right.\)
