We've had a couple of classes on the topic with geometric sequences and arithmetic series as well. I've been having a propblem finding the values to use in the sum of the geometric sequence formula.

$\displaystyle

tn=\frac{ar^n-1}{r-1}

$

8

$\displaystyle

\sum(1/3)^(k-9)

$

k=4

I`m having trouble getting the a and r values,is there any general rules you can use when looking for them in sigma notation