JEE Main & Advanced Sample Paper JEE Main - Mock Test - 24

  • question_answer
    The value of \[\underset{n\to \infty }{\mathop{\lim }}\,\sum\limits_{k=1}^{n}{\frac{1}{n+k}}\left( {{\log }_{e}}(n+k)-{{\log }_{e}}n \right)\] is equal to

    A) \[\frac{{{({{\log }_{e}}2)}^{2}}}{4}\] 

    B)        \[\frac{{{({{\log }_{e}}2)}^{2}}}{3}\]

    C) \[\frac{{{({{\log }_{e}}2)}^{2}}}{2}\] 

    D)         \[{{({{\log }_{e}}2)}^{2}}\]

    Correct Answer: C

    Solution :

    [c] \[\underset{k=1}{\mathop{\overset{n}{\mathop{\Sigma }}\,}}\,\frac{1}{n\left( 1+\frac{k}{n} \right)}.{{\log }_{e}}\left( 1+\frac{k}{n} \right)=\int\limits_{0}^{1}{\frac{{{\log }_{e}}(1+x)}{(1+x)}}dx\] \[=\int\limits_{1}^{2}{\frac{{{\log }_{e}}t}{t}}dt=\left[ \frac{{{({{\log }_{e}}t)}^{2}}}{2} \right]_{1}^{2}=\frac{{{({{\log }_{e}}2)}^{2}}}{2}\]


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