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

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