Đáp án:
$\begin{array}{l}
\sin \left( {x + \dfrac{\pi }{2}} \right) = \sin \left( { - \dfrac{\pi }{2} + 2x} \right)\\
\Rightarrow \left[ \begin{array}{l}
x + \dfrac{\pi }{2} = - \dfrac{\pi }{2} + 2x + k2\pi \\
x + \dfrac{\pi }{2} = \pi + \dfrac{\pi }{2} - 2x + k2\pi
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
x = \pi - k2\pi \\
x = \dfrac{\pi }{3} + \dfrac{{k2\pi }}{3}
\end{array} \right.\\
\Rightarrow x = \dfrac{\pi }{3} + \dfrac{{k2\pi }}{3}
\end{array}$
