Results 1 to 2 of 2

Math Help - find the value of a in this sequence

  1. #1
    Member
    Joined
    Sep 2008
    Posts
    222

    find the value of a in this sequence

    a, 3a,...
    The first term in the sequence above is a, and each term after the first is 3 times the preceding term. If the sum of the first 5 terms is 605, what is the value of a? I got 25 but the correct answer is 5. Why? THanks in advance
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Moo
    Moo is offline
    A Cute Angle Moo's Avatar
    Joined
    Mar 2008
    From
    P(I'm here)=1/3, P(I'm there)=t+1/3
    Posts
    5,618
    Thanks
    6
    Quote Originally Posted by fabxx View Post
    a, 3a,...
    The first term in the sequence above is a, and each term after the first is 3 times the preceding term. If the sum of the first 5 terms is 605, what is the value of a? I got 25 but the correct answer is 5. Why? THanks in advance
    So I guess you weren't far from the solution.
    It would be nice if you showed it after that, so that one can tell you what was wrong.

    a+3a+3^2a+3^3a+3^4a=605

    This can be factored : a(1+3+3^2+3^3+3^4)=605

    What's in brackets is a geometric sum. We know that 1+r+r^2+\dots+r^{n-1}=\frac{1-r^n}{1-r}

    Can you do it ?
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. [SOLVED] Find an example of a sequence
    Posted in the Calculus Forum
    Replies: 1
    Last Post: October 12th 2011, 04:38 PM
  2. How to find the sequence?
    Posted in the Number Theory Forum
    Replies: 13
    Last Post: October 8th 2011, 01:32 AM
  3. Find a sequence
    Posted in the Discrete Math Forum
    Replies: 1
    Last Post: February 25th 2011, 05:21 AM
  4. find limit of sequence
    Posted in the Algebra Forum
    Replies: 5
    Last Post: March 20th 2009, 05:01 AM
  5. Find Cauchy sequence
    Posted in the Calculus Forum
    Replies: 4
    Last Post: October 20th 2008, 02:05 PM

Search Tags


/mathhelpforum @mathhelpforum