Integral de $$$- \frac{\sin{\left(x \right)}}{\sin{\left(a - x \right)}}$$$ con respecto a $$$x$$$

Halla $$$\int \left(- \frac{\sin{\left(x \right)}}{\sin{\left(a - x \right)}}\right)\, dx$$$.

Reescribe el integrando:

$${\color{red}{\int{\left(- \frac{\sin{\left(x \right)}}{\sin{\left(a - x \right)}}\right)d x}}} = {\color{red}{\int{\left(- \frac{\sin{\left(x \right)}}{\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}}\right)d x}}}$$

Reescribe el numerador y separa la fracción:

$${\color{red}{\int{\left(- \frac{\sin{\left(x \right)}}{\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}}\right)d x}}} = {\color{red}{\int{\left(\frac{\left(- \sin{\left(a \right)} \sin{\left(x \right)} - \cos{\left(a \right)} \cos{\left(x \right)}\right) \sin{\left(a \right)}}{\left(\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}\right) \left(\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}\right)} + \frac{\cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}}\right)d x}}}$$

Integra término a término:

$${\color{red}{\int{\left(\frac{\left(- \sin{\left(a \right)} \sin{\left(x \right)} - \cos{\left(a \right)} \cos{\left(x \right)}\right) \sin{\left(a \right)}}{\left(\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}\right) \left(\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}\right)} + \frac{\cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}}\right)d x}}} = {\color{red}{\left(\int{\frac{\cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}} d x} + \int{\frac{\left(- \sin{\left(a \right)} \sin{\left(x \right)} - \cos{\left(a \right)} \cos{\left(x \right)}\right) \sin{\left(a \right)}}{\left(\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}\right) \left(\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}\right)} d x}\right)}}$$

Aplica la regla de la constante $$$\int c\, dx = c x$$$ con $$$c=\frac{\cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}}$$$:

$$\int{\frac{\left(- \sin{\left(a \right)} \sin{\left(x \right)} - \cos{\left(a \right)} \cos{\left(x \right)}\right) \sin{\left(a \right)}}{\left(\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}\right) \left(\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}\right)} d x} + {\color{red}{\int{\frac{\cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}} d x}}} = \int{\frac{\left(- \sin{\left(a \right)} \sin{\left(x \right)} - \cos{\left(a \right)} \cos{\left(x \right)}\right) \sin{\left(a \right)}}{\left(\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}\right) \left(\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}\right)} d x} + {\color{red}{\frac{x \cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}}}}$$

Sea $$$u=\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}$$$.

Entonces $$$du=\left(\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}\right)^{\prime }dx = \left(- \sin{\left(a \right)} \sin{\left(x \right)} - \cos{\left(a \right)} \cos{\left(x \right)}\right) dx$$$ (los pasos pueden verse »), y obtenemos que $$$\left(- \sin{\left(a \right)} \sin{\left(x \right)} - \cos{\left(a \right)} \cos{\left(x \right)}\right) dx = du$$$.

Entonces,

$$\frac{x \cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}} + {\color{red}{\int{\frac{\left(- \sin{\left(a \right)} \sin{\left(x \right)} - \cos{\left(a \right)} \cos{\left(x \right)}\right) \sin{\left(a \right)}}{\left(\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}\right) \left(\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}\right)} d x}}} = \frac{x \cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}} + {\color{red}{\int{\frac{\sin{\left(a \right)}}{u \left(\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}\right)} d u}}}$$

Aplica la regla del factor constante $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$ con $$$c=\frac{\sin{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}}$$$ y $$$f{\left(u \right)} = \frac{1}{u}$$$:

$$\frac{x \cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}} + {\color{red}{\int{\frac{\sin{\left(a \right)}}{u \left(\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}\right)} d u}}} = \frac{x \cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}} + {\color{red}{\frac{\sin{\left(a \right)} \int{\frac{1}{u} d u}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}}}}$$

La integral de $$$\frac{1}{u}$$$ es $$$\int{\frac{1}{u} d u} = \ln{\left(\left|{u}\right| \right)}$$$:

$$\frac{x \cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}} + \frac{\sin{\left(a \right)} {\color{red}{\int{\frac{1}{u} d u}}}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}} = \frac{x \cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}} + \frac{\sin{\left(a \right)} {\color{red}{\ln{\left(\left|{u}\right| \right)}}}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}}$$

Recordemos que $$$u=\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}$$$:

$$\frac{x \cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}} + \frac{\ln{\left(\left|{{\color{red}{u}}}\right| \right)} \sin{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}} = \frac{x \cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}} + \frac{\ln{\left(\left|{{\color{red}{\left(\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}\right)}}}\right| \right)} \sin{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}}$$

Por lo tanto,

$$\int{\left(- \frac{\sin{\left(x \right)}}{\sin{\left(a - x \right)}}\right)d x} = \frac{x \cos{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}} + \frac{\ln{\left(\left|{\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}}\right| \right)} \sin{\left(a \right)}}{\sin^{2}{\left(a \right)} + \cos^{2}{\left(a \right)}}$$

Simplificar:

$$\int{\left(- \frac{\sin{\left(x \right)}}{\sin{\left(a - x \right)}}\right)d x} = x \cos{\left(a \right)} + \ln{\left(\left|{\sin{\left(a - x \right)}}\right| \right)} \sin{\left(a \right)}$$

Añade la constante de integración:

$$\int{\left(- \frac{\sin{\left(x \right)}}{\sin{\left(a - x \right)}}\right)d x} = x \cos{\left(a \right)} + \ln{\left(\left|{\sin{\left(a - x \right)}}\right| \right)} \sin{\left(a \right)}+C$$

$$$\int \left(- \frac{\sin{\left(x \right)}}{\sin{\left(a - x \right)}}\right)\, dx = \left(x \cos{\left(a \right)} + \ln\left(\left|{\sin{\left(a - x \right)}}\right|\right) \sin{\left(a \right)}\right) + C$$$A