PT $\Leftrightarrow \sin 3x=\cos (\pi+2x)$
$\Leftrightarrow \sin 3x=\sin(\dfrac{\pi}{2}-\pi-2x)$
$\Leftrightarrow \sin 3x=\sin (\dfrac{-\pi}{2}-2x)$
$\Leftrightarrow 3x=\dfrac{-\pi}{2}-2x+k2\pi\Leftrightarrow x=\dfrac{-\pi}{10}+\dfrac{k2\pi}{5}$
hoặc $3x=\pi+\dfrac{\pi}{2}+2x+k2\pi\Leftrightarrow x=\dfrac{3\pi}{2}+k2\pi$
$0<x<\pi$
- TH1: $0<\dfrac{-\pi}{10}+k\dfrac{2\pi}{5}<\pi$
$\Leftrightarrow 0,25<k<2,75$
$\Rightarrow k=1; 2$
- TH2: $0<\dfrac{3\pi}{2}+k2\pi<\pi$
$\Leftrightarrow -0,75<k<-0,25$ (loại)
Vậy phương trình 2 nghiệm: $\dfrac{3\pi}{10},\dfrac{7\pi}{10}$
Tổng nghiệm: $\pi$
