$$$\cot{\left(x \right)}$$$ 的积分

求$$$\int \cot{\left(x \right)}\, dx$$$。

将余切改写为 $$$\cot\left(x\right)=\frac{\cos\left(x\right)}{\sin\left(x\right)}$$$:

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

设$$$u=\sin{\left(x \right)}$$$。

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

该积分可以改写为

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

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

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

回忆一下 $$$u=\sin{\left(x \right)}$$$:

$$\ln{\left(\left|{{\color{red}{u}}}\right| \right)} = \ln{\left(\left|{{\color{red}{\sin{\left(x \right)}}}}\right| \right)}$$

因此，

$$\int{\cot{\left(x \right)} d x} = \ln{\left(\left|{\sin{\left(x \right)}}\right| \right)}$$

加上积分常数：

$$\int{\cot{\left(x \right)} d x} = \ln{\left(\left|{\sin{\left(x \right)}}\right| \right)}+C$$

$$$\int \cot{\left(x \right)}\, dx = \ln\left(\left|{\sin{\left(x \right)}}\right|\right) + C$$$A