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Geomtric sequence agghh!

  1. Sep 9, 2008 #1
    the sum of the first n terms of an arithmetic sequence {un} is given by the
    formula sn=4n2-2n. three terms of this secuence, u2 um and u32 are conscutive terms en a geomtric sequence. find m.

    Yea, im really confused... please help :-)
  2. jcsd
  3. Sep 9, 2008 #2
    you can find tn by

    [tex]u_{n} = s_{n} - s_{n-1}[/tex]

    Then find [tex]u_{m}[/tex] by applying properties of geometric sequence. work back and find m
  4. Sep 11, 2008 #3


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    In particular, [tex]u_2= s_2- s_1[/tex]. Find [tex]s_2[/tex] and [tex]s_1[/tex] from the formula you are given and subtract. [tex]u_{32}= s_{32}- s_{31}[/tex]. Again find those and subtract. Now you know two terms of a geometric sequence you can find the term between them. [tex]\frac{u_m}{u_2}= \frac{u_{32}}{u_m}[/tex].
    Last edited by a moderator: Sep 11, 2008
  5. Sep 11, 2008 #4
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