Infinite Geometric Series I'm currently stuck on this question:
What is the value of c if $\sum_{n=1}^\infty (1 + c)^{-n}$ = 4 and c > 0?
This appears to be an infinite geometric series with a = 1 and r = $(1 + c)^{-1}$, so if I plug this all into the sum of infinite geometric series formula $S = \frac{a}{1 - r}$, then I get the following:
$4 = \frac{1}{1 - (1 + c)^{-1}}$, which eventually lets me solve c = $\frac{1}{3}$. But this answer isn't right. Can someone help me out? Thanks in advance!

HINT:
As $n$ starts with $1$ not $0$
$\displaystyle a=(1+c)^{-1}=\frac1{1+c},$ not $(1+c)^0=1$
