JEE Main & Advanced Mathematics Limits, Continuity and Differentiability Question Bank Self Evaluation Test - Continuity and Differentiability

  • question_answer
    If \[f(x)={{\log }_{x}}(In\,\,x)\], then at \[x=e,f'(x)\]equals-

    A) 0

    B) 1

    C) e

    D) 1/e

    Correct Answer: D

    Solution :

    [d] \[\because \,\,\,ln\,\,x={{\log }_{e}}x,so\] \[f(x)={{\log }_{x}}({{\log }_{e}}x)=\frac{\log (\log x)}{\log x}\] \[\Rightarrow \,\,\,f'(x)=\frac{\log x\left( \frac{1}{x\log x} \right)-\log (\log \,x).\frac{1}{x}}{{{(\log \,x)}^{2}}}\] \[\therefore \,\,\,\,f'(e)=\frac{1/e-0}{{{(1)}^{2}}}=\frac{1}{e}\]


You need to login to perform this action.
You will be redirected in 3 sec spinner