JEE Main & Advanced Mathematics Sequence & Series Question Bank Arithmetico - Geometric Series, Method of difference

  • question_answer
    \[{{2}^{1/4}}{{.4}^{1/8}}{{.8}^{1/16}}{{.16}^{1/32}}..........\]is equal to  [MNR 1984; MP PET 1998; AIEEE 2002]

    A) 1

    B) 2

    C) \[\frac{3}{2}\]

    D) \[\frac{5}{2}\]

    Correct Answer: B

    Solution :

    \[{{2}^{1/4}}{{.4}^{1/8}}{{.8}^{1/16}}{{.16}^{1/32}}.....\infty \] \[={{2}^{1/4+2/8+3/16+......}}={{2}^{S}}\], where \[S\] is given by                \[S=\frac{1}{4}+2\frac{1}{8}+3\frac{1}{16}+4\frac{1}{32}+...........\infty \] ......(i) \[\Rightarrow \]\[\frac{1}{2}S=\text{        }\frac{1}{8}+\frac{2}{16}+\frac{3}{32}+\frac{4}{64}+..........\infty \] ......(ii) Subtracting (ii) from (i), we get \[S=1\]. Hence required product\[={{2}^{1}}=2\].


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