Integration by parts
Example 1:
$$\int xe^x \, dx $$
Solution:
$$\int xe^x \, dx $$
\begin{align*}
\textup{Let}\,\, u &= x \;\;\;\;\;\;\;\; dv = e^x \,dx\\
du &= dx \;\;\;\;\;\;\;\; v = e^x \\\\
\because\int u\,dv &= uv-\int v\,du \\\\
\therefore\int xe^x \,dx &= xe^x-\int e^x\,dx \\
\int xe^x \,dx &= xe^x-e^x+C \\
\end{align*}
Example 2:
$$\int x \sin x \, dx $$
Solution:
$$\int x \sin x \, dx $$
\begin{align*}
\textup{Let}\,\, u &= x \;\;\;\;\;\;\;\; dv = \sin x \,dx\\
du &= dx \;\;\;\;\;\;\;\; v = -\cos x \\\\
\because\int u\,dv &= uv-\int v\,du \\\\
\therefore\int x \sin x \,dx &= x (-\cos x) -\int (-\cos x)\,dx \\
\int x \sin x \,dx &= -x \cos x + \int (\cos x)\,dx \\
\int x \sin x \,dx &= -x \cos x+ \sin x+C \\
\end{align*}
Example 3:
$$\int x \cos x \, dx $$
Solution:
$$\int x \cos x \, dx $$
\begin{align*}
\textup{Let}\,\, u &= x \;\;\;\;\;\;\;\; dv = \cos x \,dx\\
du &= dx \;\;\;\;\;\;\;\; v = \sin x \\\\
\because\int u\,dv &= uv-\int v\,du \\\\
\therefore\int x \cos x \,dx &= x \sin x -\int \sin x\,dx \\
\int x \cos x \,dx &= x \sin x - (-\cos x) + C \\
\int x \cos x \,dx &= x \sin x + \cos x + C \\
\end{align*}