\(\begin{array}{l}a)\,\,\sin 3x - \cos 5x = 0\\\Leftrightarrow \cos 5x=\sin 3x = \cos \left( {\frac{\pi }{2} - 3x} \right)\\\Leftrightarrow \left[ \begin{array}{l}5x = \frac{\pi }{2} - 3x + k2\pi \\5x = - \frac{\pi }{2} + 3x + k2\pi \end{array} \right.\\\Leftrightarrow \left[ \begin{array}{l}8x = \frac{\pi }{2} + k2\pi \\2x = - \frac{\pi }{2} + k2\pi \end{array} \right.\\\Leftrightarrow \left[ \begin{array}{l}x = \frac{\pi }{{16}} + \frac{{k\pi }}{4}\\x = - \frac{\pi }{4} + k\pi \end{array} \right.\,\,\,\left( {k \in Z} \right)\end{array}\)
Vậy nghiệm phương trình là: \(x=\frac{\pi }{16}+\frac{k\pi }{4} (k\in Z)\) và \(x=-\frac{\pi }{4} +k\pi, (k\in \mathbb{Z})\)
b) Điều kiện:
\(\begin{array}{l}\,\,\,\,\,\,\,\left\{ \begin{array}{l}\cos 3x \ne 0\\\cos x \ne 0\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}3x \ne \frac{\pi }{2} + k\pi \\x \ne \frac{\pi }{2} + k\pi \end{array} \right.\\\Leftrightarrow \left\{ \begin{array}{l}x \ne \frac{\pi }{6} + \frac{{k\pi }}{3}\\x \ne \frac{\pi }{2} + k\pi \end{array} \right. \Rightarrow x \ne \frac{\pi }{6} + \frac{{k\pi }}{3}\,\,\left( {k \in Z} \right)\end{array}\)
\(\begin{array}{l}\,\,\,\,\,\,\,\tan 3x\tan x = 1\\\Leftrightarrow \tan 3x = \frac{1}{{\tan x}} = \cot x =\tan \left( {\frac{\pi }{2} - x} \right)\\\Leftrightarrow 3x = \frac{\pi }{2} - x + k\pi \\\Leftrightarrow 4x = \frac{\pi }{2} + k\pi \\\Leftrightarrow x = \frac{\pi }{8} + \frac{{k\pi }}{4}\,\,\,\left( {k \in Z} \right)\,\,\,\left( {tm} \right)\end{array}\)
Vậy nghiệm phương trình là \(x=\frac{\pi }{8}+\frac{k \pi }{4}, k \in \mathbb{Z}\).
