Integraal van $$$\cos{\left(n x \right)}$$$ met betrekking tot $$$x$$$

Bepaal $$$\int \cos{\left(n x \right)}\, dx$$$.

Zij $$$u=n x$$$.

Dan $$$du=\left(n x\right)^{\prime }dx = n dx$$$ (de stappen zijn te zien »), en dan geldt dat $$$dx = \frac{du}{n}$$$.

Dus,

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

Pas de constante-veelvoudregel $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$ toe met $$$c=\frac{1}{n}$$$ en $$$f{\left(u \right)} = \cos{\left(u \right)}$$$:

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

De integraal van de cosinus is $$$\int{\cos{\left(u \right)} d u} = \sin{\left(u \right)}$$$:

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

We herinneren eraan dat $$$u=n x$$$:

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

Dus,

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

Voeg de integratieconstante toe:

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

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