a) $\cos{x} + 2 \sqrt{2} \sin{x} = m-1 \\
\Leftrightarrow \dfrac{1}{9} \cos{x} + \dfrac{2 \sqrt{2}}{9} \sin{x} = \dfrac{m-1}{9} \\
\Leftrightarrow \cos{x} \cos{\alpha} + \sin{x} \sin{\alpha} = \dfrac{m-1}{9} \ \ \ \left( \cos{\alpha} = \dfrac{1}{9} \right ) \\
\Leftrightarrow \cos \left ( x - \alpha \right ) = \dfrac{m-1}{9}$
Phương trình có nghiệm $\Leftrightarrow -1 \leq \dfrac{m-1}{9} \leq 1$
$\Leftrightarrow -9 \leq m-1 \leq 9 \\
\Leftrightarrow -8 \leq m \leq 10$