Results 1 to 3 of 3

Math Help - Arithmetic Sequence Sums to be PROVEN

  1. #1
    Newbie
    Joined
    Mar 2011
    Posts
    4

    Question Arithmetic Sequence Sums to be PROVEN

    I have a problem which I have the answer to but I don't understand why and how to prove it?

    If the sum of the first 10 terms of an a.p. is 120, sum of the first 20 =840. find sum of first 30 terms....

    I believe the answer to be 2160 but I don't understand why...
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    Joined
    Dec 2007
    From
    Ottawa, Canada
    Posts
    3,119
    Thanks
    68
    SUM = n[2a + d(n - 1)] / 2 ; n = number of terms, a = 1st term, d = common difference

    10[2a + d(10 - 1)] / 2 = 120 ; simplify: 2a + 9d = 24 [eq. 1]

    20[2a + d(20 - 1)] / 2 = 840 ; simplify: 2a + 19d = 84 [eq. 2]

    Get a and d using 2 above equations. Then apply to n = 30. OK?
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    Mar 2011
    Posts
    4
    Ah ha
    Thank You.
    I think I get it now!
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Sequence cube sums
    Posted in the Algebra Forum
    Replies: 6
    Last Post: December 3rd 2011, 04:51 AM
  2. Problem with (sums of) arithmetic sequences
    Posted in the Algebra Forum
    Replies: 20
    Last Post: April 10th 2011, 04:51 PM
  3. Replies: 2
    Last Post: August 24th 2010, 03:10 AM
  4. Sequence of partial sums is Cauchy
    Posted in the Differential Geometry Forum
    Replies: 3
    Last Post: June 5th 2009, 04:59 AM
  5. Replies: 12
    Last Post: November 15th 2006, 01:51 PM

Search Tags


/mathhelpforum @mathhelpforum