JEE Main & Advanced Mathematics Functions Question Bank Limits

  • question_answer
    The value of \[\underset{n\to \infty }{\mathop{\lim }}\,\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+...+\frac{1}{(2n-1)(2n+1)}\] is equal to [DCE 2005]

    A)                 ½

    B)                 1/3

    C)                 ¼

    D)                 None of these

    Correct Answer: A

    Solution :

                       \[\underset{n\to \infty }{\mathop{\lim }}\,\frac{1}{2}\left[ \left( 1-\frac{1}{3} \right)+\left( \frac{1}{3}-\frac{1}{5} \right)+\left( \frac{1}{5}-\frac{1}{7} \right)+.... \right.\]                                                                       \[\left. +\left( \frac{1}{(2n-1)}-\frac{1}{(2n+1)} \right) \right]\]                                 \[=\underset{n\to \infty }{\mathop{\lim }}\,\frac{1}{2}\left[ 1-\frac{1}{2n+1} \right]=\frac{1}{2}\].


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