1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: A simple sequences

  1. Oct 12, 2005 #1
    my problem is regarding sequences:

    Sum first 20 terms of a=r^k-1

    terms are 2, 4/3, 8/9,...
    and ratio is 2/3
    and r < 1 so its an inflated series
    and 2(2/3)^19 = .009021859795

    BUT it is an inflated series right? and my answer is the sum of the
    first 20 terms which has to be larger than 2 right? so where am I
    going wrong?
  2. jcsd
  3. Oct 12, 2005 #2


    User Avatar
    Homework Helper

    a=r^(k-1) is a formula for the k-th term, not for the sum of the first k terms.
    The formula for the sum is: [tex]s_n=t_{1}\frac{1-q^n}{1-q}[/tex]
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook