Cho \(\cos \alpha  =  - \frac{2}{5}\,\,\,\left( {\...

Câu hỏi: Cho \(\cos \alpha  =  - \frac{2}{5}\,\,\,\left( {\frac{\pi }{2} < \alpha  < \pi } \right)\). Khi đó \(\tan \alpha \) bằng

A. \(\frac{{\sqrt {21} }}{3}\)

B. \(-\frac{{\sqrt {21} }}{5}\)

C. \(\frac{{\sqrt {21} }}{5}\)

D. \(-\frac{{\sqrt {21} }}{2}\)