$$$\frac{\sin{\left(x \right)}}{3 \cos{\left(x \right)}}$$$ 的积分

求$$$\int \frac{\sin{\left(x \right)}}{3 \cos{\left(x \right)}}\, dx$$$。

对 $$$c=\frac{1}{3}$$$ 和 $$$f{\left(x \right)} = \frac{\sin{\left(x \right)}}{\cos{\left(x \right)}}$$$ 应用常数倍法则 $$$\int c f{\left(x \right)}\, dx = c \int f{\left(x \right)}\, dx$$$：

$${\color{red}{\int{\frac{\sin{\left(x \right)}}{3 \cos{\left(x \right)}} d x}}} = {\color{red}{\left(\frac{\int{\frac{\sin{\left(x \right)}}{\cos{\left(x \right)}} d x}}{3}\right)}}$$

设$$$u=\cos{\left(x \right)}$$$。

则$$$du=\left(\cos{\left(x \right)}\right)^{\prime }dx = - \sin{\left(x \right)} dx$$$ (步骤见»)，并有$$$\sin{\left(x \right)} dx = - du$$$。

因此，

$$\frac{{\color{red}{\int{\frac{\sin{\left(x \right)}}{\cos{\left(x \right)}} d x}}}}{3} = \frac{{\color{red}{\int{\left(- \frac{1}{u}\right)d u}}}}{3}$$

对 $$$c=-1$$$ 和 $$$f{\left(u \right)} = \frac{1}{u}$$$ 应用常数倍法则 $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$：

$$\frac{{\color{red}{\int{\left(- \frac{1}{u}\right)d u}}}}{3} = \frac{{\color{red}{\left(- \int{\frac{1}{u} d u}\right)}}}{3}$$

$$$\frac{1}{u}$$$ 的积分为 $$$\int{\frac{1}{u} d u} = \ln{\left(\left|{u}\right| \right)}$$$:

$$- \frac{{\color{red}{\int{\frac{1}{u} d u}}}}{3} = - \frac{{\color{red}{\ln{\left(\left|{u}\right| \right)}}}}{3}$$

回忆一下 $$$u=\cos{\left(x \right)}$$$:

$$- \frac{\ln{\left(\left|{{\color{red}{u}}}\right| \right)}}{3} = - \frac{\ln{\left(\left|{{\color{red}{\cos{\left(x \right)}}}}\right| \right)}}{3}$$

因此，

$$\int{\frac{\sin{\left(x \right)}}{3 \cos{\left(x \right)}} d x} = - \frac{\ln{\left(\left|{\cos{\left(x \right)}}\right| \right)}}{3}$$

加上积分常数：

$$\int{\frac{\sin{\left(x \right)}}{3 \cos{\left(x \right)}} d x} = - \frac{\ln{\left(\left|{\cos{\left(x \right)}}\right| \right)}}{3}+C$$

$$$\int \frac{\sin{\left(x \right)}}{3 \cos{\left(x \right)}}\, dx = - \frac{\ln\left(\left|{\cos{\left(x \right)}}\right|\right)}{3} + C$$$A