$$$- \sin^{3}{\left(x \right)}$$$ 的積分

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

套用常數倍法則 $$$\int c f{\left(x \right)}\, dx = c \int f{\left(x \right)}\, dx$$$，使用 $$$c=-1$$$ 與 $$$f{\left(x \right)} = \sin^{3}{\left(x \right)}$$$：

$${\color{red}{\int{\left(- \sin^{3}{\left(x \right)}\right)d x}}} = {\color{red}{\left(- \int{\sin^{3}{\left(x \right)} d x}\right)}}$$

提出一個正弦因子，將其餘部分用餘弦表示，使用公式 $$$\sin^2\left(\alpha \right)=-\cos^2\left(\alpha \right)+1$$$，其中 $$$\alpha=x$$$:

$$- {\color{red}{\int{\sin^{3}{\left(x \right)} d x}}} = - {\color{red}{\int{\left(1 - \cos^{2}{\left(x \right)}\right) \sin{\left(x \right)} d x}}}$$

令 $$$u=\cos{\left(x \right)}$$$。

則 $$$du=\left(\cos{\left(x \right)}\right)^{\prime }dx = - \sin{\left(x \right)} dx$$$ (步驟見»)，並可得 $$$\sin{\left(x \right)} dx = - du$$$。

因此，

$$- {\color{red}{\int{\left(1 - \cos^{2}{\left(x \right)}\right) \sin{\left(x \right)} d x}}} = - {\color{red}{\int{\left(u^{2} - 1\right)d u}}}$$

套用常數倍法則 $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$，使用 $$$c=-1$$$ 與 $$$f{\left(u \right)} = 1 - u^{2}$$$：

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

逐項積分：

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

配合 $$$c=1$$$，應用常數法則 $$$\int c\, du = c u$$$：

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

套用冪次法則 $$$\int u^{n}\, du = \frac{u^{n + 1}}{n + 1}$$$ $$$\left(n \neq -1 \right)$$$，以 $$$n=2$$$：

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

回顧一下 $$$u=\cos{\left(x \right)}$$$：

$${\color{red}{u}} - \frac{{\color{red}{u}}^{3}}{3} = {\color{red}{\cos{\left(x \right)}}} - \frac{{\color{red}{\cos{\left(x \right)}}}^{3}}{3}$$

因此，

$$\int{\left(- \sin^{3}{\left(x \right)}\right)d x} = - \frac{\cos^{3}{\left(x \right)}}{3} + \cos{\left(x \right)}$$

加上積分常數：

$$\int{\left(- \sin^{3}{\left(x \right)}\right)d x} = - \frac{\cos^{3}{\left(x \right)}}{3} + \cos{\left(x \right)}+C$$

$$$\int \left(- \sin^{3}{\left(x \right)}\right)\, dx = \left(- \frac{\cos^{3}{\left(x \right)}}{3} + \cos{\left(x \right)}\right) + C$$$A