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

  • question_answer
    If\[y={{\log }^{n}}x\], where \[{{\log }^{n}}\] means log log log... (repeated n time), then \[x\,log\text{ }x\text{ }log\text{ }x\text{ }lo{{g}^{2}}x\text{ }lo{{g}^{3}}x\]\[....{{\log }^{n-1}}x{{\log }^{n}}x\frac{dy}{dx}\] is equal to

    A) \[\log x\]

    B) \[{{\log }^{n}}x\]

    C) \[\frac{1}{\log \,x}\]

    D) 1

    Correct Answer: B

    Solution :

    [b] \[\because \,\,\,\,\,y={{\log }^{n}}x\] On differentiating w.r.t.x, we get \[x\log x{{\log }^{2}}x{{\log }^{3}}x....{{\log }^{n-1}}x{{\log }^{n}}x\frac{dy}{dx}\] \[=\frac{x\log x{{\log }^{2}}x{{\log }^{3}}x....{{\log }^{n-1}}x{{\log }^{n}}x.1}{x\log x{{\log }^{2}}x{{\log }^{3}}x....{{\log }^{n-1}}x}\] \[={{\log }^{n}}x\]


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