$$$6 \cos{\left(3 t \right)}$$$ 的积分

求$$$\int 6 \cos{\left(3 t \right)}\, dt$$$。

对 $$$c=6$$$ 和 $$$f{\left(t \right)} = \cos{\left(3 t \right)}$$$ 应用常数倍法则 $$$\int c f{\left(t \right)}\, dt = c \int f{\left(t \right)}\, dt$$$：

$${\color{red}{\int{6 \cos{\left(3 t \right)} d t}}} = {\color{red}{\left(6 \int{\cos{\left(3 t \right)} d t}\right)}}$$

设$$$u=3 t$$$。

则$$$du=\left(3 t\right)^{\prime }dt = 3 dt$$$ (步骤见»)，并有$$$dt = \frac{du}{3}$$$。

因此，

$$6 {\color{red}{\int{\cos{\left(3 t \right)} d t}}} = 6 {\color{red}{\int{\frac{\cos{\left(u \right)}}{3} d u}}}$$

对 $$$c=\frac{1}{3}$$$ 和 $$$f{\left(u \right)} = \cos{\left(u \right)}$$$ 应用常数倍法则 $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$：

$$6 {\color{red}{\int{\frac{\cos{\left(u \right)}}{3} d u}}} = 6 {\color{red}{\left(\frac{\int{\cos{\left(u \right)} d u}}{3}\right)}}$$

余弦函数的积分为 $$$\int{\cos{\left(u \right)} d u} = \sin{\left(u \right)}$$$：

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

回忆一下 $$$u=3 t$$$:

$$2 \sin{\left({\color{red}{u}} \right)} = 2 \sin{\left({\color{red}{\left(3 t\right)}} \right)}$$

因此，

$$\int{6 \cos{\left(3 t \right)} d t} = 2 \sin{\left(3 t \right)}$$

加上积分常数：

$$\int{6 \cos{\left(3 t \right)} d t} = 2 \sin{\left(3 t \right)}+C$$

$$$\int 6 \cos{\left(3 t \right)}\, dt = 2 \sin{\left(3 t \right)} + C$$$A