Cho

Câu hỏi: Cho $\alpha$ là góc nhọn, tính sin$\alpha$, cot$\alpha$ biết cos$\alpha$ = $\frac{2}{5}$

A. sin $\alpha$ = $\frac{\sqrt{21}}{25}$; cot $\alpha$ = $\frac{3\sqrt{21}}{21}$

B. sin $\alpha$ = $\frac{\sqrt{21}}{5}$; cot $\alpha$ = $\frac{5}{\sqrt{21}}$

C. sin $\alpha$ =  $\frac{\sqrt{21}}{3}$; cot $\alpha$ = $\frac{3}{\sqrt{21}}$

D. sin $\alpha$ =  $\frac{\sqrt{21}}{5}$; cot $\alpha$ = $\frac{2}{\sqrt{21}}$