Integral of $$$\cos{\left(10 x \right)}$$$

Find $$$\int \cos{\left(10 x \right)}\, dx$$$.

Let $$$u=10 x$$$.

Then $$$du=\left(10 x\right)^{\prime }dx = 10 dx$$$ (steps can be seen »), and we have that $$$dx = \frac{du}{10}$$$.

So,

$${\color{red}{\int{\cos{\left(10 x \right)} d x}}} = {\color{red}{\int{\frac{\cos{\left(u \right)}}{10} d u}}}$$

Apply the constant multiple rule $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$ with $$$c=\frac{1}{10}$$$ and $$$f{\left(u \right)} = \cos{\left(u \right)}$$$:

$${\color{red}{\int{\frac{\cos{\left(u \right)}}{10} d u}}} = {\color{red}{\left(\frac{\int{\cos{\left(u \right)} d u}}{10}\right)}}$$

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

$$\frac{{\color{red}{\int{\cos{\left(u \right)} d u}}}}{10} = \frac{{\color{red}{\sin{\left(u \right)}}}}{10}$$

Recall that $$$u=10 x$$$:

$$\frac{\sin{\left({\color{red}{u}} \right)}}{10} = \frac{\sin{\left({\color{red}{\left(10 x\right)}} \right)}}{10}$$

Therefore,

$$\int{\cos{\left(10 x \right)} d x} = \frac{\sin{\left(10 x \right)}}{10}$$

Add the constant of integration:

$$\int{\cos{\left(10 x \right)} d x} = \frac{\sin{\left(10 x \right)}}{10}+C$$

$$$\int \cos{\left(10 x \right)}\, dx = \frac{\sin{\left(10 x \right)}}{10} + C$$$A