$$$\tan^{3}{\left(x \right)}$$$ 的积分

求$$$\int \tan^{3}{\left(x \right)}\, dx$$$。

设$$$u=\tan{\left(x \right)}$$$。

则 $$$x=\operatorname{atan}{\left(u \right)}$$$ 且 $$$dx=\left(\operatorname{atan}{\left(u \right)}\right)^{\prime }du = \frac{du}{u^{2} + 1}$$$（步骤见»）。

因此，

$${\color{red}{\int{\tan^{3}{\left(x \right)} d x}}} = {\color{red}{\int{\frac{u^{3}}{u^{2} + 1} d u}}}$$

由于分子次数不小于分母次数，进行多项式长除法（步骤见»）:

$${\color{red}{\int{\frac{u^{3}}{u^{2} + 1} d u}}} = {\color{red}{\int{\left(u - \frac{u}{u^{2} + 1}\right)d u}}}$$

逐项积分：

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

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

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

设$$$v=u^{2} + 1$$$。

则$$$dv=\left(u^{2} + 1\right)^{\prime }du = 2 u du$$$ (步骤见»)，并有$$$u du = \frac{dv}{2}$$$。

因此，

$$\frac{u^{2}}{2} - {\color{red}{\int{\frac{u}{u^{2} + 1} d u}}} = \frac{u^{2}}{2} - {\color{red}{\int{\frac{1}{2 v} d v}}}$$

对 $$$c=\frac{1}{2}$$$ 和 $$$f{\left(v \right)} = \frac{1}{v}$$$ 应用常数倍法则 $$$\int c f{\left(v \right)}\, dv = c \int f{\left(v \right)}\, dv$$$：

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

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

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

回忆一下 $$$v=u^{2} + 1$$$:

$$\frac{u^{2}}{2} - \frac{\ln{\left(\left|{{\color{red}{v}}}\right| \right)}}{2} = \frac{u^{2}}{2} - \frac{\ln{\left(\left|{{\color{red}{\left(u^{2} + 1\right)}}}\right| \right)}}{2}$$

回忆一下 $$$u=\tan{\left(x \right)}$$$:

$$- \frac{\ln{\left(1 + {\color{red}{u}}^{2} \right)}}{2} + \frac{{\color{red}{u}}^{2}}{2} = - \frac{\ln{\left(1 + {\color{red}{\tan{\left(x \right)}}}^{2} \right)}}{2} + \frac{{\color{red}{\tan{\left(x \right)}}}^{2}}{2}$$

因此，

$$\int{\tan^{3}{\left(x \right)} d x} = - \frac{\ln{\left(\tan^{2}{\left(x \right)} + 1 \right)}}{2} + \frac{\tan^{2}{\left(x \right)}}{2}$$

加上积分常数：

$$\int{\tan^{3}{\left(x \right)} d x} = - \frac{\ln{\left(\tan^{2}{\left(x \right)} + 1 \right)}}{2} + \frac{\tan^{2}{\left(x \right)}}{2}+C$$

$$$\int \tan^{3}{\left(x \right)}\, dx = \left(- \frac{\ln\left(\tan^{2}{\left(x \right)} + 1\right)}{2} + \frac{\tan^{2}{\left(x \right)}}{2}\right) + C$$$A