JEE Main & Advanced JEE Main Paper (Held on 08-4-2019 Morning)

  • question_answer
    If\[f(x)=\frac{2-x\,\cos x}{2+x\,\cos x}\]and\[g(x)=lo{{g}_{e}}x,(x>0)\]then the value of integral\[\int\limits_{-\frac{\pi }{4}}^{\frac{\pi }{4}}{g(f(x))dx}\]is:                   [JEE Main 8-4-2019 Morning]

    A) \[{{\log }_{e}}3\]                  

    B) \[{{\log }_{e}}2\]

    C) \[{{\log }_{e}}e\]                   

    D) \[{{\log }_{e}}1\]

    Correct Answer: D

    Solution :

    \[g(f(x))=\ell n(f(x))=\ell n\left( \frac{2-x.\cos x}{2+x.\cos x} \right)\]           \[\therefore \]\[I=\int\limits_{-\pi /4}^{\pi /4}{\ell n}\left( \frac{2-x.\cos x}{2+x.\cos x} \right)dx\]           \[=\int\limits_{0}^{\pi /4}{\left( \ell n\left( \frac{2-x.\cos x}{2+x.\cos x} \right)+\ell n\left( \frac{2+x.\cos x}{2-x.\cos x} \right) \right)dx}\]           \[=\int\limits_{0}^{\pi /4}{(0)dx=0={{\log }_{e}}}(1)\]


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