Cho

C

- Hướng dẫn giải

Đáp án C.

Đặt $t=\sin 3x\Rightarrow dt=3c\text{os3xdx}\Rightarrow \left\{\begin{array}{l}x=0\\ x=\frac{\pi }{6}\end{array}\right.\to \left\{\begin{array}{l}t=0\\ t=1\end{array}\right.$

$\Rightarrow I=\underset{0}{\overset{\frac{\pi }{6}}{\int }}f\left(\sin 3x\right).c\text{os}3x.dx=\frac{1}{3}\underset{0}{\overset{1}{\int }}f\left(t\right)dt=3.$