Results 1 to 2 of 2

Math Help - Gemoetric Series

  1. #1
    Newbie
    Joined
    Oct 2008
    Posts
    16

    Gemoetric Series

    Find the sum of the first ten terms of a GP which has 3rd term 20 and 8th term 640.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Eater of Worlds
    galactus's Avatar
    Joined
    Jul 2006
    From
    Chaneysville, PA
    Posts
    3,001
    Thanks
    1
    The nth tem of a geometric series is given by

    a_{n}=a_{1}r^{n-1}

    So, with the given info we have:

    a_{3}=a_{1}r^{2}=20

    a_{8}=a_{1}r^{7}=640

    The nth partial sum is given by:

    S_{n}=a_{1}\cdot\frac{1-r^{n}}{1-r}

    So, the sum of the first 10 terms is

    S_{10}=a_{1}\cdot\frac{1-r^{10}}{1-r}


    Solve the first two equations for a_{1} \;\ and \;\ r

    Then, use them in the partial sum equation.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 5
    Last Post: October 3rd 2011, 01:12 AM
  2. Replies: 3
    Last Post: September 29th 2010, 06:11 AM
  3. Replies: 0
    Last Post: January 26th 2010, 08:06 AM
  4. Replies: 2
    Last Post: September 16th 2009, 07:56 AM
  5. Replies: 1
    Last Post: May 5th 2008, 09:44 PM

Search Tags


/mathhelpforum @mathhelpforum