KVPY Sample Paper KVPY Stream-SX Model Paper-17

  • question_answer
    If \[{{a}_{n}}=\sqrt{7+\sqrt{7+\sqrt{7+...}}}\]having a radical signs then by methods of mathematical induction which is true

    A) \[{{a}_{n}}>7\forall n\ge 1\]

    B) \[{{a}_{n}}<7\forall n\ge 1\]

    C) \[{{a}_{n}}<4\forall n\ge 1\]

    D) \[{{a}_{n}}<3\forall n\ge 1\]

    Correct Answer: B

    Solution :

    \[{{a}_{1}}=\sqrt{7}<7.\]Let \[{{a}_{m}}<7\]
    Then\[{{a}_{m+1}}=\sqrt{7+{{a}_{m}}}\Rightarrow {{a}^{2}}_{m+1}=7+{{a}_{m}}\]       
    \[<7+7<14.\]
    \[\Rightarrow \]\[{{a}_{m+1}}<\sqrt{14}<7;\]so by the principle of mathematical induction \[{{a}_{m+1}}<\sqrt{14}\forall n.\]


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