# sequences

• Mar 17th 2010, 02:34 AM
Dgphru
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!!
• Mar 17th 2010, 02:50 AM
HallsofIvy
Quote:

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.