Đáp án: \(\left[ \begin{array}{l}x = -\dfrac{π}{2} + k2π\\x = \dfrac{π}{6} + k\dfrac{2π}{3}\end{array} \right.\) `( k ∈ ZZ)`
Giải thích các bước giải:
`cos 2x = -cos (x + π/2)`
`<=> cos 2x = cos (x - π/2)`
`<=>` \(\left[ \begin{array}{l}2x = x - \dfrac{π}{2} + k2π\\2x = \dfrac{π}{2} - x + k2π\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x = -\dfrac{π}{2} + k2π\\x = \dfrac{π}{6} + k\dfrac{2π}{3}\end{array} \right.\) `( k ∈ ZZ)`
