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

  • question_answer
    If the \[{{p}^{th}}\],\[{{q}^{th}}\] and \[{{r}^{th}}\]term of a G.P. are \[a,\ b,\ c\] respectively, then \[{{a}^{q-r}}.\ {{b}^{r-p}}.\ {{c}^{p-q}}\] is equal to [Roorkee 1955, 63, 73; Pb. CET 1991, 95]

    A) 0

    B) 1

    C) \[abc\]

    D) \[pqr\]

    Correct Answer: B

    Solution :

    Let \[a,\ b,\ c,\ d\]    ?..(i) \[A{{R}^{q-1}}=b\]           ?..(ii) and \[A{{R}^{r-1}}=c\]           ?..(iii) So \[{{a}^{q-r}}{{b}^{r-p}}{{c}^{p-q}}\]\[={{\left\{ A{{R}^{p-1}} \right\}}^{q-r}}\left\{ A{{R}^{q-1}} \right\}{{\,}^{r-p}}{{\left\{ A{{R}^{r-1}} \right\}}^{p-q}}\] \[{{12}^{th}}\] \[={{A}^{0}}{{R}^{0}}=1\]. Note: Such type of questions \[i.e.\] containing terms of powers in cyclic order associated with negative sign, reduce to 1 mostly. 


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