Integral de $$$- x \cot{\left(x \right)} + x$$$

Halla $$$\int \left(- x \cot{\left(x \right)} + x\right)\, dx$$$.

Integra término a término:

$${\color{red}{\int{\left(- x \cot{\left(x \right)} + x\right)d x}}} = {\color{red}{\left(\int{x d x} - \int{x \cot{\left(x \right)} d x}\right)}}$$

Aplica la regla de la potencia $$$\int x^{n}\, dx = \frac{x^{n + 1}}{n + 1}$$$ $$$\left(n \neq -1 \right)$$$ con $$$n=1$$$:

$$- \int{x \cot{\left(x \right)} d x} + {\color{red}{\int{x d x}}}=- \int{x \cot{\left(x \right)} d x} + {\color{red}{\frac{x^{1 + 1}}{1 + 1}}}=- \int{x \cot{\left(x \right)} d x} + {\color{red}{\left(\frac{x^{2}}{2}\right)}}$$

Para la integral $$$\int{x \cot{\left(x \right)} d x}$$$, utiliza la integración por partes $$$\int \operatorname{u} \operatorname{dv} = \operatorname{u}\operatorname{v} - \int \operatorname{v} \operatorname{du}$$$.

Sean $$$\operatorname{u}=x$$$ y $$$\operatorname{dv}=\cot{\left(x \right)} dx$$$.

Entonces $$$\operatorname{du}=\left(x\right)^{\prime }dx=1 dx$$$ (los pasos pueden verse ») y $$$\operatorname{v}=\int{\cot{\left(x \right)} d x}=\ln{\left(\sin{\left(x \right)} \right)}$$$ (los pasos pueden verse »).

Entonces,

$$\frac{x^{2}}{2} - {\color{red}{\int{x \cot{\left(x \right)} d x}}}=\frac{x^{2}}{2} - {\color{red}{\left(x \cdot \ln{\left(\sin{\left(x \right)} \right)}-\int{\ln{\left(\sin{\left(x \right)} \right)} \cdot 1 d x}\right)}}=\frac{x^{2}}{2} - {\color{red}{\left(x \ln{\left(\sin{\left(x \right)} \right)} - \int{\ln{\left(\sin{\left(x \right)} \right)} d x}\right)}}$$

Esta integral no tiene una forma cerrada:

$$\frac{x^{2}}{2} - x \ln{\left(\sin{\left(x \right)} \right)} + {\color{red}{\int{\ln{\left(\sin{\left(x \right)} \right)} d x}}} = \frac{x^{2}}{2} - x \ln{\left(\sin{\left(x \right)} \right)} + {\color{red}{\left(\frac{i x^{2}}{2} - x \ln{\left(1 - e^{2 i x} \right)} + x \ln{\left(\sin{\left(x \right)} \right)} + \frac{i \operatorname{Li}_{2}\left(e^{2 i x}\right)}{2}\right)}}$$

Por lo tanto,

$$\int{\left(- x \cot{\left(x \right)} + x\right)d x} = \frac{x^{2}}{2} + \frac{i x^{2}}{2} - x \ln{\left(1 - e^{2 i x} \right)} + \frac{i \operatorname{Li}_{2}\left(e^{2 i x}\right)}{2}$$

Simplificar:

$$\int{\left(- x \cot{\left(x \right)} + x\right)d x} = \frac{x^{2} \left(1 + i\right)}{2} - x \ln{\left(1 - e^{2 i x} \right)} + \frac{i \operatorname{Li}_{2}\left(e^{2 i x}\right)}{2}$$

Añade la constante de integración:

$$\int{\left(- x \cot{\left(x \right)} + x\right)d x} = \frac{x^{2} \left(1 + i\right)}{2} - x \ln{\left(1 - e^{2 i x} \right)} + \frac{i \operatorname{Li}_{2}\left(e^{2 i x}\right)}{2}+C$$

$$$\int \left(- x \cot{\left(x \right)} + x\right)\, dx = \left(\frac{x^{2} \left(1 + i\right)}{2} - x \ln\left(1 - e^{2 i x}\right) + \frac{i \operatorname{Li}_{2}\left(e^{2 i x}\right)}{2}\right) + C$$$A