a) $\cos \left( \alpha +\pi \right) $
Mang dấu “-“ do $\pi <\alpha +\pi < \frac{ 3\pi}{2}$
b) $\tan \left( \alpha -\pi \right)=\tan \alpha $
Mang dấu “+” do $0<\alpha < \frac{ \pi}{2}\Rightarrow -\pi<\alpha-\pi<-\frac{\pi}{2}$
c) $\sin \left( \alpha +\frac{ \pi}{2} \right) =- cos \alpha $
Mang dấu “-“ do $0< \alpha < \frac{ \pi}{2}\Rightarrow \frac{\pi}{2}<\alpha+\frac{\pi}{2}<\pi$
d) $cos \left( \alpha -\frac{ 3\pi}{8} \right)= -\cos \left( \alpha + \frac{ \pi}{8}- \frac{ \pi}{2} \right) $ mang dấu “+”.