$\int\limits_{2}^{3}\frac{xdx}{(x-1)^{2}(x+1)^{3}}$
bài này dễ mà.sao lại phải đăng lên –  caigithenhi2601 24-02-13 11:47 AM
Ta phân tích như sau:
\[\frac{x}{{{{(x - 1)}^2}{{(x + 1)}^3}}} = \frac{A}{{{{(x - 1)}^2}}} + \frac{B}{{{{(x + 1)}^3}}} + \frac{C}{{(x - 1)(x + 1)}}\]
\[ = \frac{{A({x^3} + 3{x^2} + 3x + 1) + B({x^2} - 2x + 1) + C({x^3} + {x^2} - x - 1)}}{{{{(x - 1)}^2}{{(x + 1)}^3}}}\]
Từ đây, ta được hệ phương trình:
\begin{cases}A+C=0 \\ 3A+B+C=0 \\ 3A-2B-C=1 \\ A+B-C=0 \end{cases}
\[ \Leftrightarrow A = \frac{1}{8},B =  - \frac{1}{4},C =  - \frac{1}{8}\]
Khi đó, ta có:
\[I = \frac{1}{8}\int\limits_2^3 {\frac{{dx}}{{{{(x - 1)}^2}}}}  - \frac{1}{4}\int\limits_2^3 {\frac{{dx}}{{{{(x + 1)}^3}}}}  - \frac{1}{8}\int\limits_2^3 {\frac{{dx}}{{(x - 1)(x + 1)}}} \]
\[ = \left[ { - \frac{1}{8}\frac{1}{{x - 1}} + \frac{1}{8}\frac{1}{{{{(x + 1)}^2}}}} \right]\begin{cases}3 \\ 2 \end{cases} - \frac{1}{{16}}\int\limits_2^3 {\left( {\frac{1}{{x - 1}} - \frac{1}{{x + 1}}} \right)dx} \]
\[ = \frac{{65}}{{1152}} - \frac{1}{{16}}\left[ {\ln (x - 1) - \ln (x + 1)} \right]\begin{cases}3 \\ 2 \end{cases}\]
\[ = \frac{{65 - 72\ln \frac{3}{2}}}{{1152}}\]


$\int\limits_{2}^{3}x\tfrac{dx}{(x-1)^{2}(x+1)^{3}}$
$đặt  t= x+1$ $\Rightarrow dx=dt$
$x= t-1$
$x=2 \Rightarrow t=3$
$x=3\Rightarrow t=4$
$tích phân  trở  thành$ 
$\int\limits_{3}^{4}(t-1)\tfrac{dt}{t^{2}t^{3}}$
$\int\limits_{3}^{4}\frac{dt}{t^{4}}-\frac{dt}{t^{5}}$
$đơn  giản  rồi  nhé$
Đặt t= x 1 thì mẫu thức bằng (t-2)^2.t^3 chứ! –  binhnguyenhoangvu 10-05-13 05:28 PM

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