$$$\frac{1}{6 - \frac{a}{50}}$$$ 的积分

求$$$\int \frac{1}{6 - \frac{a}{50}}\, da$$$。

设$$$u=6 - \frac{a}{50}$$$。

则$$$du=\left(6 - \frac{a}{50}\right)^{\prime }da = - \frac{da}{50}$$$ (步骤见»)，并有$$$da = - 50 du$$$。

因此，

$${\color{red}{\int{\frac{1}{6 - \frac{a}{50}} d a}}} = {\color{red}{\int{\left(- \frac{50}{u}\right)d u}}}$$

对 $$$c=-50$$$ 和 $$$f{\left(u \right)} = \frac{1}{u}$$$ 应用常数倍法则 $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$：

$${\color{red}{\int{\left(- \frac{50}{u}\right)d u}}} = {\color{red}{\left(- 50 \int{\frac{1}{u} d u}\right)}}$$

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

$$- 50 {\color{red}{\int{\frac{1}{u} d u}}} = - 50 {\color{red}{\ln{\left(\left|{u}\right| \right)}}}$$

回忆一下 $$$u=6 - \frac{a}{50}$$$:

$$- 50 \ln{\left(\left|{{\color{red}{u}}}\right| \right)} = - 50 \ln{\left(\left|{{\color{red}{\left(6 - \frac{a}{50}\right)}}}\right| \right)}$$

因此，

$$\int{\frac{1}{6 - \frac{a}{50}} d a} = - 50 \ln{\left(\left|{\frac{a}{50} - 6}\right| \right)}$$

化简：

$$\int{\frac{1}{6 - \frac{a}{50}} d a} = 50 \left(- \ln{\left(\left|{a - 300}\right| \right)} + \ln{\left(50 \right)}\right)$$

加上积分常数：

$$\int{\frac{1}{6 - \frac{a}{50}} d a} = 50 \left(- \ln{\left(\left|{a - 300}\right| \right)} + \ln{\left(50 \right)}\right)+C$$

$$$\int \frac{1}{6 - \frac{a}{50}}\, da = 50 \left(- \ln\left(\left|{a - 300}\right|\right) + \ln\left(50\right)\right) + C$$$A