$$$\frac{3}{x - 4}$$$ 的积分

求$$$\int \frac{3}{x - 4}\, dx$$$。

对 $$$c=3$$$ 和 $$$f{\left(x \right)} = \frac{1}{x - 4}$$$ 应用常数倍法则 $$$\int c f{\left(x \right)}\, dx = c \int f{\left(x \right)}\, dx$$$：

$${\color{red}{\int{\frac{3}{x - 4} d x}}} = {\color{red}{\left(3 \int{\frac{1}{x - 4} d x}\right)}}$$

设$$$u=x - 4$$$。

则$$$du=\left(x - 4\right)^{\prime }dx = 1 dx$$$ (步骤见»)，并有$$$dx = du$$$。

因此，

$$3 {\color{red}{\int{\frac{1}{x - 4} d x}}} = 3 {\color{red}{\int{\frac{1}{u} d u}}}$$

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

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

回忆一下 $$$u=x - 4$$$:

$$3 \ln{\left(\left|{{\color{red}{u}}}\right| \right)} = 3 \ln{\left(\left|{{\color{red}{\left(x - 4\right)}}}\right| \right)}$$

因此，

$$\int{\frac{3}{x - 4} d x} = 3 \ln{\left(\left|{x - 4}\right| \right)}$$

加上积分常数：

$$\int{\frac{3}{x - 4} d x} = 3 \ln{\left(\left|{x - 4}\right| \right)}+C$$

$$$\int \frac{3}{x - 4}\, dx = 3 \ln\left(\left|{x - 4}\right|\right) + C$$$A