JEE Main & Advanced Mathematics Sequence & Series Question Bank Self Evaluation Test - Sequences and Series

  • question_answer
    Let a, b, c, be in A.P. with a common difference d. Then \[{{e}^{1/c}},{{e}^{b/ac}},{{e}^{1/a}}\] are in:

    A) G.P. with common ratio \[{{e}^{d}}\]

    B) G.P. with common ratio \[{{e}^{1/d}}\]

    C) G.P. with common ratio \[{{e}^{d/({{b}^{2}}-{{d}^{2}})}}\]

    D)             A.P.

    Correct Answer: C

    Solution :

    [c] a, b, c are in A.P. \[\Rightarrow \,\,\,\,2b=a+c\] Now, \[{{e}^{1/c}}\times {{e}^{1/a}}={{e}^{(a+c)/ac}}={{e}^{2b/ac}}={{({{e}^{b/ac}})}^{2}}\] \[\therefore \,\,\,{{e}^{1/c}},\,\,{{e}^{b/ac}},{{e}^{1/a}}\] in GP. with common ratio \[=\frac{{{e}^{b/ac}}}{{{e}^{1/c}}}={{e}^{(b-a)/ac}}={{e}^{d/(b-d)(b+d)}}\] \[={{e}^{d/({{b}^{2}}-{{d}^{2}})}}\]  [ \[\because \] a, b, c are in A.P. with common difference d \[\therefore \,\,\,\,b-a=c-b=d\]]


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