Câu 28:

Cho $\tan \alpha = \sqrt 5 \,\,\left( {\pi < \alpha < \frac{{3\pi }}{2}} \right)$. Khi đó $\cos \alpha$ bằng:

Câu 28:

Cho \tan \alpha&nbsp; = \sqrt 5 \,\,\left( {\pi&nbsp; &lt; \alpha&nbsp; &lt; \frac{{3\pi }}{2}} \right). Khi đó $\cos \alpha$ bằng: