Pt $\Leftrightarrow cos(x + \frac{\pi }{3})= \frac{-\sqrt{2}}{2}= cos \frac{3\pi }{4}$$\Leftrightarrow x + \frac{\pi }{3}=\pm \frac{3\pi }{4} + k2\pi $
$\Leftrightarrow x=\frac{5\pi }{12} + k2\pi ,x= \frac{-13\pi }{12} + k2\pi , k\in Z $
Do $x \in ( 0 ; \pi ) $
TH1 : $x = \frac{5\pi }{12}+ k2\pi $
Có $0 < \frac{5\pi }{12} + k2\pi <\pi $
$\Rightarrow k =0 \Rightarrow x= \frac{5\pi }{12}$
TH2 : $x= \frac{-13\pi }{12} + k2\pi$ tương tự ta đc $x = \frac{11\pi }{12} $