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

  • question_answer
    If \[a,\,b,\,c\] are three unequal numbers such that \[a,\,b,\,c\] are in A.P. and b - a, c - b, a are in G.P., then a : b : c is [UPSEAT 2001]

    A) 1 : 2 : 3

    B) 2: 3 : 1

    C) 1 : 3 : 2

    D) 3 : 2 : 1

    Correct Answer: A

    Solution :

    a, b, c are in A.P. Þ  a + c = 2b         ....(i) Also, \[b-a,\,c-b,\,a\] are in G.P. Þ \[{{(c-b)}^{2}}=(b-a)a\] Þ \[(b-a)\,(c-b)\,=(b-a)\,a\]\[(\because \,\,c-b=b-a\] as a, b, c are in A.P.) Þ \[c-b=a\]  \[(\because \,\,a\ne b)\] Þ \[b=c-a\] ?..(ii) From (i) and (ii), \[a=\frac{b}{2}\text{ and }c=\frac{3b}{2}\] \ \[a:b:c::\frac{b}{2}:b:\frac{3b}{2}\] Þ \[a:b:c::1:2:3\].


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