Đáp án:
20) $B. \dfrac{\pi}{4} + k\dfrac{\pi}{2}$
22) $B. x = k\pi;\, x = \dfrac{\pi}{4} + k\dfrac{\pi}{2}$
Giải thích các bước giải:
$\begin{array}{l}20)\,\,\cos^2x = \dfrac{1}{2}\\ \Leftrightarrow \dfrac{1 + \cos2x}{2} = \dfrac{1}{2}\\ \Leftrightarrow \cos2x = 0\\ \Leftrightarrow x = \dfrac{\pi}{4} + k\dfrac{\pi}{2}\quad (k \in \Bbb Z)\\ 22)\,\,\sin3x = \sin x\\ \Leftrightarrow \left[\begin{array}{l}3x = x + k2\pi\\3x = \pi - x + k2\pi\end{array}\right.\\ \Leftrightarrow \left[\begin{array}{l}x = k\pi\\x = \dfrac{\pi}{4} + k\dfrac{\pi}{2}\end{array}\right.\quad (k \in \Bbb Z) \end{array}$
