Integral de $$$\cos{\left(3 x \right)}$$$

Encontre $$$\int \cos{\left(3 x \right)}\, dx$$$.

Seja $$$u=3 x$$$.

Então $$$du=\left(3 x\right)^{\prime }dx = 3 dx$$$ (veja os passos »), e obtemos $$$dx = \frac{du}{3}$$$.

Portanto,

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

Aplique a regra do múltiplo constante $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$ usando $$$c=\frac{1}{3}$$$ e $$$f{\left(u \right)} = \cos{\left(u \right)}$$$:

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

A integral do cosseno é $$$\int{\cos{\left(u \right)} d u} = \sin{\left(u \right)}$$$:

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

Recorde que $$$u=3 x$$$:

$$\frac{\sin{\left({\color{red}{u}} \right)}}{3} = \frac{\sin{\left({\color{red}{\left(3 x\right)}} \right)}}{3}$$

Portanto,

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

Adicione a constante de integração:

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

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