Geomatic Sequence

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In a geomatic sequence, the seventh term exceeds the fifth term by 1920. Find the sum of the first eleven terms if the common ratio of the sequence is 2.

  • Geomatic Sequence -

    that's geometric sequence

    T7 = T5*r^2 = T5+1920
    r = 2, so

    4*T5 = T5 + 1920
    3T5 = 1920
    T5 = 640

    640 = 2^4 * 40

    So, a=40 and the sequence is

    40 80 160 320 640 1280 2560 5120 10240 20480 40960 ...

    Note that 2560 = 640 + 1920

  • Geomatic Sequence -

    ar^6 - ar^4=1920
    64a-16a=1920
    48a=1920
    a = 40

    sum(11) = 40(2^11 - 1)/(2-1) = 81880

  • Geomatic Sequence -

    oops - good catch, Reiny. You actually answered the question :-)

  • Geomatic Sequence -

    find the fifth term in the sequence a1=-4a2=8a3=-16 can you guys help me

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