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

  • question_answer
    If \[\int{\frac{1}{x+{{x}^{5}}}dx=f(x)+c}\], then the value of \[\int{\frac{{{x}^{4}}}{x+{{x}^{5}}}dx}\] is [DCE 2005]

    A)                 \[\log x-f(x)+c\]

    B)                 \[f(x)+\log x+c\]

    C)                 \[f(x)-\log x+c\]

    D)                 None of these

    Correct Answer: A

    Solution :

    \[\frac{{{x}^{4}}dx}{x+{{x}^{5}}}=\int{\frac{({{x}^{4}}+1)dx}{x+{{x}^{5}}}}\]\[=\int{\frac{({{x}^{4}}+1)dx}{x+{{x}^{5}}}}\]\[-\int{\frac{dx}{x+{{x}^{5}}}}\]                       \[=\int{\frac{({{x}^{4}}+1)dx}{x(1+{{x}^{4}})}}-\int{\frac{dx}{x({{x}^{4}}+1)}}\]\[=\int{\frac{dx}{x}}-\int{\frac{dx}{x+{{x}^{5}}}}\]                    \[=\log x-f(x)-{{c}_{2}}+{{c}_{1}}=\log x-f(x)+c\]                                 Where \[{{c}_{1}}-{{c}_{2}}=c=\]a new constant.


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