Đáp án:
$\begin{array}{l}
\cos \left( {2x + {{60}^0}} \right) = \cos x\\
\Rightarrow \left[ \begin{array}{l}
2x + {60^0} = x + k{.360^0}\\
2x + {60^0} = - x + k{.360^0}
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
x = - {60^0} + k{.360^0}\\
x = - {20^0} + k{.120^0}
\end{array} \right.
\end{array}$