# Thread: Summing a series problem

1. ## Summing a series problem

Hi There, i am trying to sum up a sequence of numbers using the
summing rule for
1 + a^1 + a^2 + a^3 + a^4 +... + a^n = (a^n - 1/(a-1),

for this sequence:

1/9 + 1/9^2 + 1/9^3 + .. + 1/9^n.

Is it as simple as just inverting the formula ie :
((a^n - 1/(a-1))^-1 ?? What about the 1 that doesn't exist at the
beginning, normally i would subract a 1 after my calculation but if I
do that here the answer will be negative.

Any help appreciated
Many thanks

2. the sum is $\frac{a^{n+1}-1}{a-1}$ and put $a=\frac19.$