JEE Main & Advanced Mathematics Functions Question Bank Limits

  • question_answer
    \[\underset{n\to \infty }{\mathop{\lim }}\,\left[ \frac{1}{{{n}^{3}}+1}+\frac{4}{{{n}^{3}}+1}+\frac{9}{{{n}^{3}}+1}+........+\frac{{{n}^{2}}}{{{n}^{3}}+1} \right]=\]

    A)                 \[1\]

    B)                 2/3

    C)                 1/3

    D)                 \[0\]

    Correct Answer: C

    Solution :

                       Given limit \[=\underset{n\to \infty }{\mathop{\lim }}\,\,\,\frac{{{1}^{2}}+{{2}^{2}}+{{3}^{2}}+.....+{{n}^{2}}}{1+{{n}^{3}}}=\underset{n\to \infty }{\mathop{\lim }}\,\,\,\frac{\Sigma {{n}^{2}}}{1+{{n}^{3}}}\]            \[=\underset{n\to \infty }{\mathop{\lim }}\,\,\,\frac{1}{6}\frac{n\,(n+1)\,(2n+1)}{1+{{n}^{3}}}\]\[=\underset{n\to \infty }{\mathop{\lim }}\,\,\frac{1}{6}\frac{\left( 1+\frac{1}{n} \right)\,\left( 2+\frac{1}{n} \right)}{\left( \frac{1}{{{n}^{3}}}+1 \right)}\]                                 \[=\frac{1}{6}\,.\,1\,.\,\frac{2}{(1)}=\left( \frac{1}{3} \right).\]


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