JEE Main & Advanced Mathematics Indefinite Integrals Question Bank Integration by Parts

  • question_answer
    \[\int_{{}}^{{}}{\frac{\log x\ dx}{{{x}^{3}}}=}\]    [Roorkee 1986]

    A)                 \[\frac{1}{4{{x}^{2}}}(2\log x-1)+c\]

    B)                 \[-\frac{1}{4{{x}^{2}}}(2\log x+1)+c\]

    C)                 \[\frac{1}{4{{x}^{2}}}(2\log x+1)+c\]

    D)                 \[\frac{1}{4{{x}^{2}}}(1-2\log x)+c\]

    Correct Answer: B

    Solution :

                    \[\int_{{}}^{{}}{\frac{\log x}{{{x}^{3}}}dx=\int_{{}}^{{}}{{{x}^{-3}}\log x\ dx}}\]                            \[=-\frac{\log x}{2{{x}^{2}}}+\int_{{}}^{{}}{\frac{1}{x}.\frac{1}{2{{x}^{2}}}+c=-\frac{\log x}{2{{x}^{2}}}+\frac{1}{2}.\frac{{{x}^{-2}}}{-2}+c}\]                   \[=-\frac{\log x}{2{{x}^{2}}}-\frac{1}{4{{x}^{2}}}+c=-\frac{1}{4{{x}^{2}}}(2\log x+1)+c\].


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