Integraal van $$$\sin^{2}{\left(\frac{2 x}{3} \right)}$$$

Bepaal $$$\int \sin^{2}{\left(\frac{2 x}{3} \right)}\, dx$$$.

Zij $$$u=\frac{2 x}{3}$$$.

Dan $$$du=\left(\frac{2 x}{3}\right)^{\prime }dx = \frac{2 dx}{3}$$$ (de stappen zijn te zien »), en dan geldt dat $$$dx = \frac{3 du}{2}$$$.

De integraal wordt

$${\color{red}{\int{\sin^{2}{\left(\frac{2 x}{3} \right)} d x}}} = {\color{red}{\int{\frac{3 \sin^{2}{\left(u \right)}}{2} d u}}}$$

Pas de constante-veelvoudregel $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$ toe met $$$c=\frac{3}{2}$$$ en $$$f{\left(u \right)} = \sin^{2}{\left(u \right)}$$$:

$${\color{red}{\int{\frac{3 \sin^{2}{\left(u \right)}}{2} d u}}} = {\color{red}{\left(\frac{3 \int{\sin^{2}{\left(u \right)} d u}}{2}\right)}}$$

Pas de machtsreductieformule $$$\sin^{2}{\left(\alpha \right)} = \frac{1}{2} - \frac{\cos{\left(2 \alpha \right)}}{2}$$$ toe met $$$\alpha= u $$$:

$$\frac{3 {\color{red}{\int{\sin^{2}{\left(u \right)} d u}}}}{2} = \frac{3 {\color{red}{\int{\left(\frac{1}{2} - \frac{\cos{\left(2 u \right)}}{2}\right)d u}}}}{2}$$

Pas de constante-veelvoudregel $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$ toe met $$$c=\frac{1}{2}$$$ en $$$f{\left(u \right)} = 1 - \cos{\left(2 u \right)}$$$:

$$\frac{3 {\color{red}{\int{\left(\frac{1}{2} - \frac{\cos{\left(2 u \right)}}{2}\right)d u}}}}{2} = \frac{3 {\color{red}{\left(\frac{\int{\left(1 - \cos{\left(2 u \right)}\right)d u}}{2}\right)}}}{2}$$

Integreer termgewijs:

$$\frac{3 {\color{red}{\int{\left(1 - \cos{\left(2 u \right)}\right)d u}}}}{4} = \frac{3 {\color{red}{\left(\int{1 d u} - \int{\cos{\left(2 u \right)} d u}\right)}}}{4}$$

Pas de constantenregel $$$\int c\, du = c u$$$ toe met $$$c=1$$$:

$$- \frac{3 \int{\cos{\left(2 u \right)} d u}}{4} + \frac{3 {\color{red}{\int{1 d u}}}}{4} = - \frac{3 \int{\cos{\left(2 u \right)} d u}}{4} + \frac{3 {\color{red}{u}}}{4}$$

Zij $$$v=2 u$$$.

Dan $$$dv=\left(2 u\right)^{\prime }du = 2 du$$$ (de stappen zijn te zien »), en dan geldt dat $$$du = \frac{dv}{2}$$$.

Dus,

$$\frac{3 u}{4} - \frac{3 {\color{red}{\int{\cos{\left(2 u \right)} d u}}}}{4} = \frac{3 u}{4} - \frac{3 {\color{red}{\int{\frac{\cos{\left(v \right)}}{2} d v}}}}{4}$$

Pas de constante-veelvoudregel $$$\int c f{\left(v \right)}\, dv = c \int f{\left(v \right)}\, dv$$$ toe met $$$c=\frac{1}{2}$$$ en $$$f{\left(v \right)} = \cos{\left(v \right)}$$$:

$$\frac{3 u}{4} - \frac{3 {\color{red}{\int{\frac{\cos{\left(v \right)}}{2} d v}}}}{4} = \frac{3 u}{4} - \frac{3 {\color{red}{\left(\frac{\int{\cos{\left(v \right)} d v}}{2}\right)}}}{4}$$

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

$$\frac{3 u}{4} - \frac{3 {\color{red}{\int{\cos{\left(v \right)} d v}}}}{8} = \frac{3 u}{4} - \frac{3 {\color{red}{\sin{\left(v \right)}}}}{8}$$

We herinneren eraan dat $$$v=2 u$$$:

$$\frac{3 u}{4} - \frac{3 \sin{\left({\color{red}{v}} \right)}}{8} = \frac{3 u}{4} - \frac{3 \sin{\left({\color{red}{\left(2 u\right)}} \right)}}{8}$$

We herinneren eraan dat $$$u=\frac{2 x}{3}$$$:

$$- \frac{3 \sin{\left(2 {\color{red}{u}} \right)}}{8} + \frac{3 {\color{red}{u}}}{4} = - \frac{3 \sin{\left(2 {\color{red}{\left(\frac{2 x}{3}\right)}} \right)}}{8} + \frac{3 {\color{red}{\left(\frac{2 x}{3}\right)}}}{4}$$

Dus,

$$\int{\sin^{2}{\left(\frac{2 x}{3} \right)} d x} = \frac{x}{2} - \frac{3 \sin{\left(\frac{4 x}{3} \right)}}{8}$$

Voeg de integratieconstante toe:

$$\int{\sin^{2}{\left(\frac{2 x}{3} \right)} d x} = \frac{x}{2} - \frac{3 \sin{\left(\frac{4 x}{3} \right)}}{8}+C$$

$$$\int \sin^{2}{\left(\frac{2 x}{3} \right)}\, dx = \left(\frac{x}{2} - \frac{3 \sin{\left(\frac{4 x}{3} \right)}}{8}\right) + C$$$A