JEE Main & Advanced Mathematics Sequence & Series Question Bank Self Evaluation Test - Sequences and Series

  • question_answer
    If \[{{a}_{1}},{{a}_{2}}.....,{{a}_{n}}\] are positive real numbers whose product is a fixed number c, then the minimum value of \[{{a}_{1}}+{{a}_{2}}+.....+{{a}_{n-1}}+2{{a}_{n}}\] is

    A) \[n{{(2c)}^{1/n}}\]

    B) \[(n+1){{c}^{1/n}}\]

    C) \[2n{{c}^{1/n}}\]

    D) \[(n+1){{(2c)}^{1/n}}\]

    Correct Answer: A

    Solution :

    [a] We have \[({{a}_{1}}+{{a}_{2}}+....+{{a}_{n-1}}+2{{a}_{n}})/n\ge {{({{a}_{1}}{{a}_{2}}....{{a}_{n-1}}2{{a}_{n}})}^{1/n}}\] \[\Rightarrow \,\,\,{{a}_{1}}+{{a}_{2}}+{{a}_{3}}+......+{{a}_{n-1}}+2{{a}_{n}}\ge n{{(2c)}^{1/n}}\]


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