# Math Help - sequences

1. ## sequences

Hi, sorry can't seem to figure this out..please help me ..?

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
Hi, sorry can't seem to figure this out..please help me ..?

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!!
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.