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Sequences and series help .

  1. Jul 23, 2012 #1
    Sequences and series help.....

    1. The problem statement, all variables and given/known data
    3+3a+3a^2+...........∞ is = to 45/8 where a>0,then a is...?


    3. The attempt at a solution
    since it is a g.p so using
    S=(a(rn-1))/(r-1) for r>1
    ive all the values except for "n"..can someone help...:/
     
  2. jcsd
  3. Jul 23, 2012 #2

    eumyang

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    Re: Sequences and series help.....

    Wrong formula. This is an infinite geometric series, so use
    [tex]a + ar + ar^2 + ar^3 + ... = \frac{a}{1 - r}[/tex]
     
  4. Jul 23, 2012 #3
    Re: Sequences and series help.....

    thanks..
     
  5. Jul 23, 2012 #4
    Re: Sequences and series help.....

    n is the number of terms, so in this case there are infinite terms.
    As n tends to infinity, what does r^n tend to? (Assuming -1<r<1)
    This gets you to the formula that eumyang posted.
     
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