$3$\displaystyle{\int f $ \cos x $ \sin xdx}$ 　

$3$\displaystyle{ \cos x=t }$ 　とおくと,
$3$\displaystyle{ \frac{\hspace{2} dt \hspace{2}}{\hspace{2} dx \hspace{2}}=\displaystyle{ - \sin x } }$
$3$\displaystyle{ dx=- \frac{\hspace{2}1\hspace{2}}{\hspace{2} \sin x \hspace{2}}dt }$
よって,与式　　
$3$\displaystyle{=- \int f $t$ \sin x\cdot \frac{\hspace{2}1\hspace{2}}{\hspace{2} \sin x \hspace{2}}dt}$
$3$\displaystyle{=- \int f $t$ dt }$