Question
Evalute : $\int \frac{x}{(x-1)^2(x+2)} d x$

Answer

Let $I =\int \frac{x}{(x-1)^2(x+2)} d x$
Let $\frac{x}{(x-1)^2(x+2)}=\frac{A}{x-1}+\frac{B}{(x-1)^2}+\frac{C}{x+2}$
$
\therefore x = A ( x -1)( x +2)+ B ( x +2)+ C ( x -1)^2
$
Put $x-1=0$, i.e. $x=1$, we get
$
\begin{aligned}
& 1= A (0)(3)+ B (3)+ C (0) \\
& \therefore B =\frac{1}{3}
\end{aligned}
$
Put $x+2=0$, i.e. $x=-2$, we get
$
\begin{aligned}
& -2= A (-3)(0)+ B (0)+ C (9) \\
& \therefore C =-\frac{2}{9}
\end{aligned}
$
Put $x =-1$, we get,
$
-1=A(-2)(1)+B(1)+C(4)
$
But $B =\frac{1}{3}$ and $C =-\frac{2}{9}$
$
\begin{aligned}
& \therefore-1=-2 A+\frac{1}{3}-\frac{8}{9} \\
& \therefore 2 A=-\frac{5}{9}+1=\frac{4}{9} \quad \therefore A=\frac{2}{9}
\end{aligned}
$
$
\begin{aligned}
& \therefore \frac{x}{(x-1)^2(x+2)}=\frac{(2 / 9)}{x-1}+\frac{(1 / 3)}{(x-1)^2}+\frac{(-2 / 9)}{x+2} \\
& \therefore I=\int\left[\frac{(2 / 9)}{x-1}+\frac{(1 / 3)}{(x-1)^2}+\frac{(-2 / 9)}{x+2}\right] d x \\
& \quad=\frac{2}{9} \int \frac{1}{x-1} d x+\frac{1}{3} \int(x-1)^{-2} d x-\frac{2}{9} \int \frac{1}{x+2} d x
\end{aligned}
$
$
=\frac{2}{9} \log |x-1|+\frac{1}{3} \cdot \frac{(x-1)^{-1}}{-1}-\frac{2}{9} \log |x+2|+c
$
$
=\frac{2}{9} \log \left|\frac{x-1}{x+2}\right|-\frac{1}{3(x-1)}+c \text {. }
$

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