`#huy`
`a)cos3x=(\sqrt{3})/2`
`<=>cos3x=3` với `cos3=(\sqrt{3})/2`
`<=>`\(\left[ \begin{array}{l}3x=3+k2\pi\\3x=-3+k2\pi\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=\dfrac{3}{2}+\dfrac{k2\pi}{2}\\x=-\dfrac{3}{2}+\dfrac{k2\pi}{2}\end{array} \right.\) `(k\inZZ)`