Đáp án:
\(\left[ \begin{array}{l}
x = - \dfrac{{255}}{2} + 180k\\
x = \dfrac{{15}}{2} + 180k
\end{array} \right.\)
Giải thích các bước giải:
\(\begin{array}{l}
\sqrt 2 \cos \left( { - 2x - 120} \right) = - 1\\
\to \cos \left( { - 2x - 120} \right) = - \dfrac{1}{{\sqrt 2 }}\\
\to \left[ \begin{array}{l}
- 2x - 120 = 135 + 360k\\
- 2x - 120 = - 135 + 360k
\end{array} \right.\\
\to \left[ \begin{array}{l}
- 2x = 255 + 360k\\
- 2x = - 15 + 360k
\end{array} \right.\\
\to \left[ \begin{array}{l}
x = - \dfrac{{255}}{2} + 180k\\
x = \dfrac{{15}}{2} + 180k
\end{array} \right.\left( {k \in Z} \right)
\end{array}\)
