Đáp án + Giải thích các bước giải:
`a.`
`cosx=1/2`
`<=>cosx=cos``pi/3`
`<=>`\(\left[ \begin{array}{l}x=\dfrac{\pi}{3}+k2\pi\\x=-\dfrac{\pi}{3}+k2\pi\end{array} \right.\)`(kinZZ)`
`b.`
`cos3x=cos``(6pi)/7`
`<=>`\(\left[ \begin{array}{l}3x=\dfrac{6\pi}{7}+k2\pi\\3x=-\dfrac{6\pi}{7}+k2\pi\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=\dfrac{2\pi}{7}+\dfrac{k2\pi}{3}\\x=-\dfrac{2\pi}{7}+\dfrac{k2\pi}{3}\end{array} \right.\)`(kinZZ)`
