JEE Main & Advanced Mathematics Definite Integration Question Bank Fundamental definite integration, Definite integration by substitution

  • question_answer
    If \[\int_{0}^{1}{x\log \left( 1+\frac{x}{2} \right)}\,dx=a+b\log \frac{2}{3},\] then              [SCRA 1986]

    A)                 \[a=\frac{3}{2},\,\,\,b=\frac{3}{2}\]        

    B)                 \[a=\frac{3}{4},\,\,\,b=-\frac{3}{4}\]

    C)                 \[a=\frac{3}{4},\,\,\,b=\frac{3}{2}\]        

    D)                 \[a=b\]

    Correct Answer: C

    Solution :

               Integrate it by parts taking \[\log \left( 1+\frac{x}{2} \right)\]as first function \[=\left[ \log \left( 1+\frac{x}{2} \right)\frac{{{x}^{2}}}{2} \right]_{0}^{2}-\int_{0}^{1}{\frac{1}{1+\frac{x}{2}}\frac{1}{2}\frac{{{x}^{2}}}{2}}dx\]                    \[=\frac{1}{2}\log \frac{3}{2}-\frac{1}{2}\int_{0}^{1}{\frac{{{x}^{2}}}{x+2}dx}\]                    \[=\frac{1}{2}\log \frac{3}{2}-\frac{1}{2}\left[ \frac{1}{2}-2+4\log 3-4\log 2 \right]=\frac{3}{4}+\frac{3}{2}\log \frac{2}{3}\]                                 On comparing with the given value \[a=\frac{3}{4},b=\frac{3}{2}\].


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