Bài 3.14 trang 166 SBT giải tích 12

Đề bài

\(\int {x\ln \left( {x + 1} \right)dx} \) bằng

A. \(\left( {\dfrac{{{x^2}}}{2} – 1} \right)\ln \left( {x + 1} \right) + \dfrac{1}{4}{\left( {x – 1} \right)^2} + C\)

B. \(\left( {\dfrac{{{x^2}}}{2} – 1} \right)\ln \left( {x + 1} \right) – \dfrac{1}{2}{\left( {x – 1} \right)^2} + C\)

C. \(\left( {\dfrac{{{x^2}}}{2} – \dfrac{1}{2}} \right)\ln \left( {x + 1} \right) – \dfrac{1}{4}{\left( {x – 1} \right)^2} + C\)

D. \(\left( {\dfrac{{{x^2}}}{2} + 1} \right)\ln \left( {x + 1} \right) – \dfrac{1}{4}{\left( {x – 1} \right)^2} + C\)

Lời giải chi tiết

Đặt \(\left\{ \begin{array}{l}u = \ln \left( {x + 1} \right)\\dv = xdx\end{array} \right.\) \( \Rightarrow \left\{ \begin{array}{l}du = \dfrac{1}{{x + 1}}dx\\v = \dfrac{{{x^2}}}{2}\end{array} \right.\)

Khi đó \(\int {x\ln \left( {x + 1} \right)dx} \)\( = \dfrac{{{x^2}}}{2}\ln \left( {x + 1} \right) – \dfrac{1}{2}\int {\dfrac{{{x^2}}}{{x + 1}}dx} \) \( = \dfrac{{{x^2}}}{2}\ln \left( {x + 1} \right) – \dfrac{1}{2}\int {\left( {x – 1 + \dfrac{1}{{x + 1}}} \right)dx} \)

\( = \dfrac{{{x^2}}}{2}\ln \left( {x + 1} \right) – \dfrac{{{x^2}}}{4} + \dfrac{1}{2}x – \dfrac{1}{2}\ln \left( {x + 1} \right) + C\) \( = \left( {\dfrac{{{x^2}}}{2} – \dfrac{1}{2}} \right)\ln \left( {x + 1} \right) – \dfrac{1}{4}\left( {{x^2} – 2x + 1} \right) + C’\)

\( = \left( {\dfrac{{{x^2}}}{2} – \dfrac{1}{2}} \right)\ln \left( {x + 1} \right) – \dfrac{1}{4}{\left( {x – 1} \right)^2} + C’\)

Chọn C.

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