Sử dụng nguyên hàm từng phần:
\(I=\int x.sin\dfrac{x}{3}dx\) \(\Rightarrow\) đặt \(\left\{{}\begin{matrix}u=x\\dv=sin\dfrac{x}{3}dx\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}du=dx\\x=-3cos\dfrac{x}{3}\end{matrix}\right.\)
\(\Rightarrow I=-3x.cos\dfrac{x}{3}+3\int cos\dfrac{x}{3}dx=-3x.cos\dfrac{x}{3}+9sin\dfrac{x}{3}+C\)