Results 1 to 5 of 5

Math Help - confused! geometric sum

  1. #1
    Newbie
    Joined
    Oct 2006
    Posts
    9

    confused! geometric sum

    determine the sum of the following geometrec sereis.

    -1/32+ 1/16...........+ 256

    well I know that I would use the formula:
    sn=a (1-r^n)/1-r

    a=-1/32
    r=
    n=

    okay this were I get lost can you expalin to me what I nedd to do next? thanks for your help!
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Super Member
    earboth's Avatar
    Joined
    Jan 2006
    From
    Germany
    Posts
    5,830
    Thanks
    123
    Quote Originally Posted by frogsrcool View Post
    determine the sum of the following geometrec sereis.
    -1/32+ 1/16...........+ 256
    well I know that I would use the formula:
    sn=a (1-r^n)/1-r
    a=-1/32
    r=
    n=
    ...
    Hi,

    are you certain that your serie starts with -1/32? (I presume that this is a typo:

    1/32+ 1/16...........+ 256 = 2^(-5)(1 + 2 + 4 + ...+2^13)
    So:
    a=1/32
    r=2
    n=13

    thus s_13 = 16383/32 nearly 512

    EB
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Grand Panjandrum
    Joined
    Nov 2005
    From
    someplace
    Posts
    14,972
    Thanks
    4
    Quote Originally Posted by earboth View Post
    Hi,

    are you certain that your serie starts with -1/32? (I presume that this is a typo:
    The common ratio could be -2, the number of terms is right for the sign
    of 256 to be +:

    -1/32, 1/16, -1/8, 1/4, -1/2, 1, -2, 4, -8, 16, -32, 64, -128, 256

    RonL
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Super Member
    earboth's Avatar
    Joined
    Jan 2006
    From
    Germany
    Posts
    5,830
    Thanks
    123

    2nd attempt

    Hi,

    OK, with CaptBlack's answer I understand your problem:

    Your serie is: -1/32, 1/16, -1/8, 1/4, -1/2, 1, -2, 4, -8, 16, -32, 64, -128, 256

    To get the result there are two different ways:

    1) Put together two summands:
    -1/32 + 1/16 = 1/32
    -1/8 + ¼ = 1/8
    -1/2 + 1 = ½
    ...
    you get a final serie: 1/32, 1/8, 1/2, 2, 8, 32, 128
    (for confirmation only: 170.65625 = 5461/32)

    2) Split your serie into two series. The first one contains all negative summands, the second one the positive summands. Calculate the sums separately. Add the sums (be aware that one sum is negative!)
    (for confirmation only: -170.65625 + 341.3125 = 170.65625)

    EB
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Newbie
    Joined
    Oct 2006
    Posts
    9

    Talking Thanks!!!

    hey I used the formula and it worked out so thanks alot! I understand what I am doing! I am very grateful!
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 6
    Last Post: May 23rd 2011, 02:36 AM
  2. Confused with some geometric problems
    Posted in the Pre-Calculus Forum
    Replies: 1
    Last Post: May 12th 2010, 10:32 PM
  3. Replies: 2
    Last Post: January 10th 2010, 08:45 AM
  4. Geometric Progression or Geometric Series
    Posted in the Math Topics Forum
    Replies: 8
    Last Post: October 8th 2009, 07:31 AM
  5. Replies: 2
    Last Post: January 23rd 2007, 08:47 AM

Search Tags


/mathhelpforum @mathhelpforum