Nếu

D

- Hướng dẫn giải

Đáp án D

Ta có 

$$\begin{gathered} \underset{0}{\overset{\pi /2}{\int }}{\cos }^{n}x\sin xdx=-\underset{0}{\overset{\pi /2}{\int }}{\cos }^{n}xd\left(\cos x\right) \\ ={\left.-\frac{1}{n+1}{\cos }^{n+1}\left(x\right)\right|}_0^{\pi /2}=\frac{1}{n+1}=\frac{1}{4} \\ \Rightarrow n=3 \end{gathered}$$