Integralen av $$$\frac{1}{- x^{2} + x}$$$

Bestäm $$$\int \frac{1}{- x^{2} + x}\, dx$$$.

Utför partialbråksuppdelning (stegen kan ses »):

$${\color{red}{\int{\frac{1}{- x^{2} + x} d x}}} = {\color{red}{\int{\left(- \frac{1}{x - 1} + \frac{1}{x}\right)d x}}}$$

Integrera termvis:

$${\color{red}{\int{\left(- \frac{1}{x - 1} + \frac{1}{x}\right)d x}}} = {\color{red}{\left(\int{\frac{1}{x} d x} - \int{\frac{1}{x - 1} d x}\right)}}$$

Integralen av $$$\frac{1}{x}$$$ är $$$\int{\frac{1}{x} d x} = \ln{\left(\left|{x}\right| \right)}$$$:

$$- \int{\frac{1}{x - 1} d x} + {\color{red}{\int{\frac{1}{x} d x}}} = - \int{\frac{1}{x - 1} d x} + {\color{red}{\ln{\left(\left|{x}\right| \right)}}}$$

Låt $$$u=x - 1$$$ vara.

Då $$$du=\left(x - 1\right)^{\prime }dx = 1 dx$$$ (stegen kan ses »), och vi har att $$$dx = du$$$.

Alltså,

$$\ln{\left(\left|{x}\right| \right)} - {\color{red}{\int{\frac{1}{x - 1} d x}}} = \ln{\left(\left|{x}\right| \right)} - {\color{red}{\int{\frac{1}{u} d u}}}$$

Integralen av $$$\frac{1}{u}$$$ är $$$\int{\frac{1}{u} d u} = \ln{\left(\left|{u}\right| \right)}$$$:

$$\ln{\left(\left|{x}\right| \right)} - {\color{red}{\int{\frac{1}{u} d u}}} = \ln{\left(\left|{x}\right| \right)} - {\color{red}{\ln{\left(\left|{u}\right| \right)}}}$$

Kom ihåg att $$$u=x - 1$$$:

$$\ln{\left(\left|{x}\right| \right)} - \ln{\left(\left|{{\color{red}{u}}}\right| \right)} = \ln{\left(\left|{x}\right| \right)} - \ln{\left(\left|{{\color{red}{\left(x - 1\right)}}}\right| \right)}$$

Alltså,

$$\int{\frac{1}{- x^{2} + x} d x} = \ln{\left(\left|{x}\right| \right)} - \ln{\left(\left|{x - 1}\right| \right)}$$

Lägg till integrationskonstanten:

$$\int{\frac{1}{- x^{2} + x} d x} = \ln{\left(\left|{x}\right| \right)} - \ln{\left(\left|{x - 1}\right| \right)}+C$$

$$$\int \frac{1}{- x^{2} + x}\, dx = \left(\ln\left(\left|{x}\right|\right) - \ln\left(\left|{x - 1}\right|\right)\right) + C$$$A