`~rai~`
\(\sin(3x+20^\circ)=\dfrac{\sqrt{3}}{2}\\\Leftrightarrow \sin\left(3x+\dfrac{\pi}{9}\right)=\dfrac{\sqrt{3}}{2}\\\Leftrightarrow \left[\begin{array}{I}3x+\dfrac{\pi}{9}=\dfrac{\pi}{3}+k2\pi\\3x+\dfrac{\pi}{9}=\pi-\dfrac{\pi}{3}+k2\pi\end{array}\right.\\\Leftrightarrow \left[\begin{array}{I}3x=\dfrac{2\pi}{9}+k2\pi\\3x=\dfrac{5\pi}{9}+k2\pi\end{array}\right.\\\Leftrightarrow \left[\begin{array}{I}x=\dfrac{2\pi}{27}+k\dfrac{2\pi}{3}\\x=\dfrac{5\pi}{27}+k\dfrac{2\pi}{3}.\end{array}\right.\quad(k\in\mathbb{Z})\\\text{Vậy S=}\left\{\dfrac{2\pi}{27}+k\dfrac{2\pi}{3};\dfrac{5\pi}{27}+k\dfrac{2\pi}{3}\Big|k\in\mathbb{Z}\right\}.\)
