Đáp án + Giải thích các bước giải:
`3.`
`cos3x=-1/2`
`<=>cos3x=cos((2pi)/3)`
`<=>`\(\left[ \begin{array}{l}3x=\dfrac{2\pi}{3}+k2\pi\\3x=-\dfrac{2\pi}{3}+k2\pi\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=\dfrac{2\pi}{9}+\dfrac{k2\pi}{3}\\x=-\dfrac{2\pi}{9}+\dfrac{k2\pi}{3}\end{array} \right.\)`(kinZZ)`
`4.`
`cos(5x)=sqrt2/2`
`<=>cos5x=cos((pi)/4)`
`<=>`\(\left[ \begin{array}{l}5x=\dfrac{\pi}{4}+k2\pi\\5x=-\dfrac{\pi}{4}+k2\pi\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=\dfrac{\pi}{20}+\dfrac{k2\pi}{5}\\x=-\dfrac{\pi}{20}+\dfrac{k2\pi}{5}\end{array} \right.\)`(kinZZ)`
