$$$4 \sin^{2}{\left(\theta \right)}$$$ 的積分

求$$$\int 4 \sin^{2}{\left(\theta \right)}\, d\theta$$$。

套用常數倍法則 $$$\int c f{\left(\theta \right)}\, d\theta = c \int f{\left(\theta \right)}\, d\theta$$$，使用 $$$c=4$$$ 與 $$$f{\left(\theta \right)} = \sin^{2}{\left(\theta \right)}$$$：

$${\color{red}{\int{4 \sin^{2}{\left(\theta \right)} d \theta}}} = {\color{red}{\left(4 \int{\sin^{2}{\left(\theta \right)} d \theta}\right)}}$$

套用降冪公式 $$$\sin^{2}{\left(\alpha \right)} = \frac{1}{2} - \frac{\cos{\left(2 \alpha \right)}}{2}$$$，令 $$$\alpha=\theta$$$:

$$4 {\color{red}{\int{\sin^{2}{\left(\theta \right)} d \theta}}} = 4 {\color{red}{\int{\left(\frac{1}{2} - \frac{\cos{\left(2 \theta \right)}}{2}\right)d \theta}}}$$

套用常數倍法則 $$$\int c f{\left(\theta \right)}\, d\theta = c \int f{\left(\theta \right)}\, d\theta$$$，使用 $$$c=\frac{1}{2}$$$ 與 $$$f{\left(\theta \right)} = 1 - \cos{\left(2 \theta \right)}$$$：

$$4 {\color{red}{\int{\left(\frac{1}{2} - \frac{\cos{\left(2 \theta \right)}}{2}\right)d \theta}}} = 4 {\color{red}{\left(\frac{\int{\left(1 - \cos{\left(2 \theta \right)}\right)d \theta}}{2}\right)}}$$

逐項積分：

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

配合 $$$c=1$$$，應用常數法則 $$$\int c\, d\theta = c \theta$$$：

$$- 2 \int{\cos{\left(2 \theta \right)} d \theta} + 2 {\color{red}{\int{1 d \theta}}} = - 2 \int{\cos{\left(2 \theta \right)} d \theta} + 2 {\color{red}{\theta}}$$

令 $$$u=2 \theta$$$。

則 $$$du=\left(2 \theta\right)^{\prime }d\theta = 2 d\theta$$$ (步驟見»)，並可得 $$$d\theta = \frac{du}{2}$$$。

該積分可改寫為

$$2 \theta - 2 {\color{red}{\int{\cos{\left(2 \theta \right)} d \theta}}} = 2 \theta - 2 {\color{red}{\int{\frac{\cos{\left(u \right)}}{2} d u}}}$$

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

$$2 \theta - 2 {\color{red}{\int{\frac{\cos{\left(u \right)}}{2} d u}}} = 2 \theta - 2 {\color{red}{\left(\frac{\int{\cos{\left(u \right)} d u}}{2}\right)}}$$

餘弦函數的積分為 $$$\int{\cos{\left(u \right)} d u} = \sin{\left(u \right)}$$$：

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

回顧一下 $$$u=2 \theta$$$：

$$2 \theta - \sin{\left({\color{red}{u}} \right)} = 2 \theta - \sin{\left({\color{red}{\left(2 \theta\right)}} \right)}$$

因此，

$$\int{4 \sin^{2}{\left(\theta \right)} d \theta} = 2 \theta - \sin{\left(2 \theta \right)}$$

加上積分常數：

$$\int{4 \sin^{2}{\left(\theta \right)} d \theta} = 2 \theta - \sin{\left(2 \theta \right)}+C$$

$$$\int 4 \sin^{2}{\left(\theta \right)}\, d\theta = \left(2 \theta - \sin{\left(2 \theta \right)}\right) + C$$$A