# Math Help - sequences

1. ## sequences

1+3=4
1+3+9=13
1+3+9 +27 = 40
and so on..

find an expression for this sequence

any guidance would help soo much .. so lost!!

2. Originally Posted by Dgphru
Those are powers of three so this is a geometric series. The sum of the geometric series $\sum_{i= 0}^n ar^{n}= a\frac{r^{n+1}- 1}{r- 1}$. Here, a= 1, r= 3.