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For a geometric series, S4/S8= 1/17, determine the first three terms of the series.

  • math -

    Sn = a(1-r^n)/(1-r), so

    (1-r^4)/(1-r^8) = 1/17
    r^4 = 1 or 16
    1 is no good, so r=2

    this will hold, regardless of the first term, T1. Letting T1=1, we have

    1,2,4,8,16,32,64,128...
    S4 = 15
    S8 = 255
    S4/S8 = 1/17

  • math -

    why (1-r^4)/(1-r^8)=1/17 then I can get r^4=1or6

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