$$$\frac{x}{x - 1}$$$ 的积分

求$$$\int \frac{x}{x - 1}\, dx$$$。

改写并拆分该分式:

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

逐项积分：

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

应用常数法则 $$$\int c\, dx = c x$$$，使用 $$$c=1$$$：

$$\int{\frac{1}{x - 1} d x} + {\color{red}{\int{1 d x}}} = \int{\frac{1}{x - 1} d x} + {\color{red}{x}}$$

设$$$u=x - 1$$$。

则$$$du=\left(x - 1\right)^{\prime }dx = 1 dx$$$ (步骤见»)，并有$$$dx = du$$$。

因此，

$$x + {\color{red}{\int{\frac{1}{x - 1} d x}}} = x + {\color{red}{\int{\frac{1}{u} d u}}}$$

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

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

回忆一下 $$$u=x - 1$$$:

$$x + \ln{\left(\left|{{\color{red}{u}}}\right| \right)} = x + \ln{\left(\left|{{\color{red}{\left(x - 1\right)}}}\right| \right)}$$

因此，

$$\int{\frac{x}{x - 1} d x} = x + \ln{\left(\left|{x - 1}\right| \right)}$$

加上积分常数：

$$\int{\frac{x}{x - 1} d x} = x + \ln{\left(\left|{x - 1}\right| \right)}+C$$

$$$\int \frac{x}{x - 1}\, dx = \left(x + \ln\left(\left|{x - 1}\right|\right)\right) + C$$$A