$$$\frac{\tan{\left(x \right)}}{\sec{\left(x \right)}}$$$ 的积分

求$$$\int \frac{\tan{\left(x \right)}}{\sec{\left(x \right)}}\, dx$$$。

设$$$u=\sec{\left(x \right)}$$$。

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

所以，

$${\color{red}{\int{\frac{\tan{\left(x \right)}}{\sec{\left(x \right)}} d x}}} = {\color{red}{\int{\frac{1}{u^{2}} d u}}}$$

应用幂法则 $$$\int u^{n}\, du = \frac{u^{n + 1}}{n + 1}$$$ $$$\left(n \neq -1 \right)$$$，其中 $$$n=-2$$$：

$${\color{red}{\int{\frac{1}{u^{2}} d u}}}={\color{red}{\int{u^{-2} d u}}}={\color{red}{\frac{u^{-2 + 1}}{-2 + 1}}}={\color{red}{\left(- u^{-1}\right)}}={\color{red}{\left(- \frac{1}{u}\right)}}$$

回忆一下 $$$u=\sec{\left(x \right)}$$$:

$$- {\color{red}{u}}^{-1} = - {\color{red}{\sec{\left(x \right)}}}^{-1}$$

因此，

$$\int{\frac{\tan{\left(x \right)}}{\sec{\left(x \right)}} d x} = - \frac{1}{\sec{\left(x \right)}}$$

化简：

$$\int{\frac{\tan{\left(x \right)}}{\sec{\left(x \right)}} d x} = - \cos{\left(x \right)}$$

加上积分常数：

$$\int{\frac{\tan{\left(x \right)}}{\sec{\left(x \right)}} d x} = - \cos{\left(x \right)}+C$$

$$$\int \frac{\tan{\left(x \right)}}{\sec{\left(x \right)}}\, dx = - \cos{\left(x \right)} + C$$$A