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

  • question_answer
    If \[,a,\ b,\,c\] be in G.P. and \[a+x,\ b+x,\ c+x\] in H.P., then the value of \[x\] is (\[a,\ b,\ c\] are distinct numbers)

    A) \[c\]

    B) \[b\]

    C) \[a\]

    D) None of these

    Correct Answer: B

    Solution :

    \[a+x,\ b+x,\ c+x\] are in H.P. \[\Rightarrow \] \[b+x=\frac{2(a+x)(c+x)}{(a+x)+(c+x)}\] \[\Rightarrow \] \[(b+x)(a+c+2x)=2(a+x)(c+x)\] \[\Rightarrow \] \[(a+c+2b)x+2{{x}^{2}}+ab+bc=2ac+2x(a+c)+2{{x}^{2}}\] \[\Rightarrow \] \[x(c+a-2b)=bc+ab-2ac\] \[\Rightarrow \] \[x(c+a-2b)=bc+ab-2{{b}^{2}}\]   (\[\because \ a,\ b,\ c\] are in G.P.) Þ\[x(c+a-2b)=b(c+a-2b)\]Þ\[x=b\], if \[c+a-2b\ne 0\].


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