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

  • question_answer
    If a, b, and c are in A.P ., and p and p' are, respectively, A.M. and G.M. between a and b while q, q' are, respectively, the A,M. and G M. between b and c, then

    A) \[{{p}^{2}}+{{q}^{2}}=p{{'}^{2}}+q{{'}^{2}}\]

    B) \[pq=p'q'\]

    C) \[{{p}^{2}}-{{q}^{2}}=p{{'}^{2}}-q{{'}^{2}}\]

    D) None of these

    Correct Answer: C

    Solution :

    [c] \[2b=a+c;\] a, p, b, q, c, are in  A.P. Hence \[p=\frac{a+b}{2}\] and \[q=\frac{b+c}{2}\] Again, a, p', b, q' and c = are in G.P. Hence, \[p'=\sqrt{ab}\] and \[q'=\sqrt{bc}\] \[{{p}^{2}}-{{q}^{2}}=\frac{(a-c)(a+c+2b)}{4}\] \[=\frac{(a-c)(2b+2b)}{4}\]    \[[\because \,\,a+c=2b]\] \[=(a-c)\,b=ab-bc=p{{'}^{2}}-q{{'}^{2}}\]


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