<=> (1 - cosx). cosx = 0 <=> cos x =1 hoặc cosx = 0
+) cosx = 0 <=> x = \(\frac{\pi}{2}+k\pi\) ; x \(\in\left[\pi;3\pi\right]\) => \(\pi\le\frac{\pi}{2}+k\pi\le3\pi\) <=> 1 \(\le\) 1/2 + k \(\le\) 3 <=> 1/2 \(\le\) k \(\le\) 2,5 ; k nguyên nên k = 1;2
=> x = \(\frac{3\pi}{2};\frac{5\pi}{2}\) đối chiếu đk sinx \(\ge\) 0 => x = \(\frac{5\pi}{2}\)
+) cosx = 1 <=> x = \(k2\pi\) ; x \(\in\left[\pi;3\pi\right]\) => x = \(2\pi\) (T/m đk sinx\(\ge\) 0)
Vậy PT có nghiệm là x = \(\frac{5\pi}{2}\); x = \(2\pi\)
