1) \( x \in \left( -\frac{\pi}{2}; \pi \right) \) b) \( \cos x = -\frac{1}{4} \), \( x \in \left( -\frac{\pi}{2}; \pi \right) \) \( x \in (0; 5\pi) \) c) \( \cos 2x = \frac{2}{5} \) \( \Rightarrow x \in \left( -\frac{\pi}{2}; \frac{\pi}{2} \right) \) \( x \in \left( -\pi; 3\pi \right) \)

vẽ hình ( có xoắn ốc ah)
----- Nội dung ảnh -----
1) \( x \in \left( -\frac{\pi}{2}; \pi \right) \)

b) \( \cos x = -\frac{1}{4} \), \( x \in \left( -\frac{\pi}{2}; \pi \right) \)

\( x \in (0; 5\pi) \)

c) \( \cos 2x = \frac{2}{5} \) \( \Rightarrow x \in \left( -\frac{\pi}{2}; \frac{\pi}{2} \right) \)

\( x \in \left( -\pi; 3\pi \right) \)