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

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

Let $$$u=\pi x$$$.

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

The integral can be rewritten as

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

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

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

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

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

Recall that $$$u=\pi x$$$:

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

Therefore,

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

Add the constant of integration:

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

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