JEE Main & Advanced Mathematics Functions Question Bank Limits

  • question_answer
    If \[{{S}_{n}}=\sum\limits_{k=1}^{n}{{{a}_{k}}}\]and\[\underset{n\to \infty }{\mathop{\lim }}\,{{a}_{n}}=a,\]then \[\underset{n\to \infty }{\mathop{\lim }}\,\frac{{{S}_{n+1}}-{{S}_{n}}}{\sqrt{\sum\limits_{k=1}^{n}{k}}}\]is equal to

    A)                 0

    B)                 a

    C)                 \[\sqrt{2}a\]

    D)                 \[2a\]

    Correct Answer: A

    Solution :

                       We have \[\underset{n\to \infty }{\mathop{\lim }}\,\,\frac{{{S}_{n+1}}-{{S}_{n}}}{\sqrt{\sum\limits_{k=1}^{n}{k}}}=\underset{n\to \infty }{\mathop{\lim }}\,\,\frac{{{a}_{n+1}}}{\sqrt{\frac{n\,(n+1)}{2}}}=0\]                                 (Since\[n\to \infty ,\,\text{numerator }\to a\text{ while}\,\,\text{denominator }\to \infty \text{)}\]


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