Biết cos α = − 7 /15 , π < α < 3 π 2 . Khi đó

\(\cos \alpha = - \frac{7}{{15}},\pi < \alpha < \frac{{3\pi }}{2}\)

a) \({\sin ^2}\alpha + {\cos ^2}\alpha = 1 \Rightarrow {\sin ^2}\alpha = \frac{{176}}{{225}}\).

b) Vì \(\pi < \alpha < \frac{{3\pi }}{2} \Rightarrow \sin \alpha < 0 \Rightarrow \sin \alpha = - \frac{{\sqrt {176} }}{{15}}\).

c)\(\tan \alpha = \frac{{\sin \alpha }}{{\cos \alpha }} = \frac{{\sqrt {176} }}{7}\).

d) \(\cot \alpha = \frac{{\cos \alpha }}{{\sin \alpha }} = \frac{7}{{\sqrt {176} }}\).

Đáp án: a) Đúng;   b) Sai;   c) Đúng; d) Sai.