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

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

A. \(\frac{{\sqrt {21} }}{2}\)

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

C. \( - \frac{{\sqrt {21} }}{4}\)

D. \(\frac{{\sqrt {21} }}{4}\)