Integral of $$$\frac{\cos{\left(2 x \right)}}{2}$$$

Find $$$\int \frac{\cos{\left(2 x \right)}}{2}\, dx$$$.

Apply the constant multiple rule $$$\int c f{\left(x \right)}\, dx = c \int f{\left(x \right)}\, dx$$$ with $$$c=\frac{1}{2}$$$ and $$$f{\left(x \right)} = \cos{\left(2 x \right)}$$$:

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

Let $$$u=2 x$$$.

Then $$$du=\left(2 x\right)^{\prime }dx = 2 dx$$$ (steps can be seen »), and we have that $$$dx = \frac{du}{2}$$$.

Thus,

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

Apply the constant multiple rule $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$ with $$$c=\frac{1}{2}$$$ and $$$f{\left(u \right)} = \cos{\left(u \right)}$$$:

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

The integral of the cosine is $$$\int{\cos{\left(u \right)} d u} = \sin{\left(u \right)}$$$:

$$\frac{{\color{red}{\int{\cos{\left(u \right)} d u}}}}{4} = \frac{{\color{red}{\sin{\left(u \right)}}}}{4}$$

Recall that $$$u=2 x$$$:

$$\frac{\sin{\left({\color{red}{u}} \right)}}{4} = \frac{\sin{\left({\color{red}{\left(2 x\right)}} \right)}}{4}$$

Therefore,

$$\int{\frac{\cos{\left(2 x \right)}}{2} d x} = \frac{\sin{\left(2 x \right)}}{4}$$

Add the constant of integration:

$$\int{\frac{\cos{\left(2 x \right)}}{2} d x} = \frac{\sin{\left(2 x \right)}}{4}+C$$

$$$\int \frac{\cos{\left(2 x \right)}}{2}\, dx = \frac{\sin{\left(2 x \right)}}{4} + C$$$A