Câu 18: $\sin x + 3 - m = 0 \Leftrightarrow \sin x = m - 3$
Vì $-1\le \sin x\le 1$ nên để phương trình có nghiệm thì $-1\le m-3\le 1$
$\Rightarrow 2\le m\le 4$
Chọn $C$
Câu 19:
$\begin{array}{l}
{\left( {\sin x - \cos x} \right)^2} = 2\\
\Leftrightarrow {\left[ {\sqrt 2 \sin \left( {x - \dfrac{\pi }{4}} \right)} \right]^2} = 2\\
\Leftrightarrow 2{\sin ^2}\left( {x - \dfrac{\pi }{4}} \right) = 2\\
\Leftrightarrow \sin \left( {x - \dfrac{\pi }{4}} \right) = \pm 1\\
\Leftrightarrow \left[ \begin{array}{l}
x - \dfrac{\pi }{4} = \dfrac{\pi }{2} + k2\pi \\
x - \dfrac{\pi }{4} = - \dfrac{\pi }{2} + k2\pi
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
x = \dfrac{{3\pi }}{4} + k2\pi \\
x = - \dfrac{\pi }{4} + k2\pi
\end{array} \right.\\
\Rightarrow x = - \dfrac{\pi }{4} + k\pi \left( {k \in \mathbb{Z}} \right)\\
S = \left\{ { - \dfrac{\pi }{4} + k\pi |k \in \mathbb{Z}} \right\}\\
\to C
\end{array}$
