\(\operatorname{Tìm} \int x \cos 2 x \mathrm{~d} x\)

\(\text { Đặt }\left\{\begin{array} { l }
{ u = x } \\
{ \mathrm { d } v = \operatorname { c o s } 2 x \mathrm { d } x }
\end{array} \Rightarrow \left\{\begin{array}{l}
\mathrm{d} u=\mathrm{d} x \\
v=\frac{1}{2} \sin 2 x
\end{array}\right.\right.\)

\(\text { Khi đó } I=\int x \cos 2 x \mathrm{~d} x=\frac{1}{0} x \sin 2 x-\frac{1}{2} \int \sin 2 x \mathrm{~d} x=\frac{1}{0} x \sin 2 x+\frac{1}{4} \cos 2 x+C\)