JEE Main & Advanced Mathematics Differentiation Question Bank Self Evaluation Test - Limits and Derivatives

  • question_answer
    If \[\frac{d}{dx}\left( \frac{1+{{x}^{4}}+{{x}^{8}}}{1+{{x}^{2}}+{{x}^{4}}} \right)=a{{x}^{3}}+bx,\] then

    A) \[a=4,b=2\]

    B) \[a=4,b=-2\]

    C) \[a=-2,b=4\]

    D) None of these

    Correct Answer: B

    Solution :

    [b] We have, \[\frac{d}{dx}\left[ \frac{(1+{{x}^{2}}+{{x}^{4}})(1-{{x}^{2}}+{{x}^{4}})}{(1+{{x}^{2}}+{{x}^{4}})} \right]=a{{x}^{3}}+bx\] \[\Rightarrow \frac{d}{dx}(1-{{x}^{2}}+{{x}^{4}})=a{{x}^{3}}+bx\] \[\Rightarrow -2x+4{{x}^{3}}=a{{x}^{3}}+bx\] \[\Rightarrow a=4\] and \[b=-2.\]


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