JEE Main & Advanced Mathematics Sequence & Series Question Bank Relation between AP., GP. and HP.

  • question_answer
    If \[{{b}^{2}},\,{{a}^{2}},\,{{c}^{2}}\] are in A.P., then \[a+b,\,b+c,\,c+a\] will be in [AMU 1974]

    A) A.P.

    B) G.P.

    C) H.P.

    D) None of these

    Correct Answer: C

    Solution :

    Given that \[{{b}^{2}},\ {{a}^{2}},\ {{c}^{2}}\] are in A.P. Therefore   \[{{a}^{2}}-{{b}^{2}}={{c}^{2}}-{{a}^{2}}\] \[\Rightarrow \]\[(a-b)(a+b)=(c-a)(c+a)\] \[\Rightarrow \] \[\frac{a-b}{c+a}=\frac{c-a}{a+b}\]\[\Rightarrow \]\[\frac{b-a+c-c}{(c+a)(b+c)}=\frac{a+b-b-c}{(b+c)(a+b)}\] \[\Rightarrow \] \[\frac{1}{b+c}-\frac{1}{a+b}=\frac{1}{c+a}-\frac{1}{b+c}\] \[\Rightarrow \] \[\frac{1}{a+b},\ \frac{1}{b+c},\ \frac{1}{c+a}\] are in A.P. Hence \[(a+b),\ (b+c),\ (c+a)\] are in H.P.


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