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

  • question_answer
    If the harmonic mean between \[a\] and \[b\] be \[H\], then \[\frac{H+a}{H-a}+\frac{H+b}{H-b}=\] [AMU 1998]

    A) 4

    B) 2

    C) 1

    D) \[a+b\]

    Correct Answer: B

    Solution :

    Putting \[H=\frac{2ab}{a+b}\] \[\frac{H+a}{H-a}+\frac{H+b}{H-b}=\frac{2({{H}^{2}}-ab)}{(H-a)(H-b)}=\frac{2\left[ \frac{4ab}{{{(a+b)}^{2}}}-ab \right]}{\left[ \frac{4ab}{{{(a+b)}^{2}}}-ab \right]}=2\]. Trick: Let \[a=1,\ H=\frac{1}{2}\] and \[b=\frac{1}{3}\], then \[\frac{H+a}{H-a}+\frac{H+b}{H-b}=\frac{3/2}{-1/2}+\frac{5/6}{1/6}=2\].


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