JEE Main & Advanced Mathematics Sequence & Series Question Bank Harmonic Progression

  • question_answer
    If \[\frac{{{a}^{n+1}}+{{b}^{n+1}}}{{{a}^{n}}+{{b}^{n}}}\] be the harmonic mean between \[a\] and \[b\], then the value of \[n\] is [Assam  PET 1986]

    A) 1

    B) \[-1\]

    C) 0

    D) 2

    Correct Answer: B

    Solution :

    We have \[\frac{{{a}^{n+1}}+{{b}^{n+1}}}{{{a}^{n}}+{{b}^{n}}}=\frac{2ab}{a+b}\] \[\Rightarrow \]\[{{a}^{n+2}}+a{{b}^{n+1}}+b{{a}^{n+1}}+{{b}^{n+2}}=2{{a}^{n+1}}b+2{{b}^{n+1}}a\] \[\Rightarrow \]\[{{a}^{n+1}}(a-b)={{b}^{n+1}}(a-b)\] or \[{{\left( \frac{a}{b} \right)}^{n+1}}=(1)={{\left( \frac{a}{b} \right)}^{0}}\] Hence\[n=-1\].


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