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

  • question_answer
    Three numbers are in G.P. such that their sum is 38 and their product is 1728. The greatest number among them is [UPSEAT 2004]

    A) 18

    B) 16

    C) 14

    D) None of these

    Correct Answer: A

    Solution :

    Let three number of G.P. i.e., \[a,\,ar,\,a{{r}^{2}}\] \[a+ar+a{{r}^{2}}=38\] and \[{{a}^{3}}{{r}^{3}}=1728\]          ?..(i) \[a(1+r+{{r}^{2}})=38\]                  ?..(ii) From equation (i) \[ar=12\] Þ \[a=\frac{12}{r}\] From equation (ii) \[\frac{12}{r}(1+r+{{r}^{2}})=38\] on solving \[r=2/3\] and \[a=\frac{12\times 3}{2}=18\] Required number 18, 12, 8 Maximum  number  i.e., 18.


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