\(sin3x+sin\left(5x-\dfrac{\pi}{6}\right)=0.\\ TXD:D=R.\\ \Leftrightarrow sin3x=-sin\left(5x-\dfrac{\pi}{6}\right).\\ \Leftrightarrow sin3x=sin\left(\dfrac{\pi}{6}+5x\right).\\ \Leftrightarrow\left[{}\begin{matrix}3x=\dfrac{\pi}{6}+5x+k2\pi.\\3x=\pi-\dfrac{\pi}{6}-5x+k2\pi.\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}-2x=\dfrac{\pi}{6}+k2\pi\\8x=\dfrac{5}{6}\pi+k2\pi.\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{-1}{12}\pi-k\pi.\\x=\dfrac{5}{48}\pi+\dfrac{k\pi}{4}.\end{matrix}\right.\)
Lời giải:
\(\sin 3x+\sin (5x-\frac{\pi}{6})=0\)
\(\Leftrightarrow \sin (5x-\frac{\pi}{6})=-\sin 3x=\sin (-3x)\)
\(\Leftrightarrow 5x-\frac{\pi}{6}=-3x+2k\pi\) hoặc $5x-\frac{\pi}{6}=\pi +3x+2k\pi$ với $k$ nguyên
$\Leftrightarrow x=\frac{1}{8}(2k+\frac{1}{6})\pi$ hoặc $x=\frac{1}{2}(\frac{7}{6}+2k)\pi$ với $k$ nguyên.
