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March 29, 2017

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The sum of the first four terms of a G.P.is 30 and that of the last four terms is 960. If the first and the last term of the G.P. are 2 and 512 respectively, find the the common ratio.

  • math - ,

    first term is 2
    so a = 2
    last term is 512
    ar^(n-1) = 512
    2r^(n-1) = 512
    r^(n-1) = 256

    a + ar + ar^2 + ar^3 = 30 , but a=2
    2(1 + r + r^2 + r^3) = 30
    (1 + r + r^2 + r^3) = 15 (equation #1)


    ar^(n-1) + ar^(n-2)+ar^(n-3)+ar^(n-4) = 960 , remember a=2
    r^(n-1) + r^(n-2)+r^(n-3)+r^(n-4) = 480
    r^(n-4)(r^3 + r^2 + r + 1) = 480 (equation #2)

    divide #2 by #1 ----> r^(n-4) = 480/15 = 32

    now r^(n-1) / r^(n-4) = 256/32 = 8

    r^3 = 8
    r = 2

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