Với \(0 < α < \frac{\pi}{2}\) ta có: \(\sin \alpha > 0,\;\;\cos \alpha > 0,\;tan\alpha > 0,\;\;\cot \alpha > 0.\)
a) Ta có: \(0 < \alpha < \pi \Rightarrow \alpha - \pi < 0 \Rightarrow \sin \left( {\alpha - \pi } \right) < 0.\)
b) Ta có: \(\cos \left( {\frac{{3\pi }}{2} - \alpha } \right) = \cos \left( {\pi + \frac{\pi }{2} - \alpha } \right) = - \cos \left( {\frac{\pi }{2} - \alpha } \right) = - \sin\alpha < 0.\)
c) Ta có: \(\tan \left( {\alpha + \pi } \right) = \tan \alpha >0.\)
d) Ta có: \(\cot \left( {\frac{\pi }{2} + \alpha } \right) = - \tan \alpha<0.\)
