The sum of the first n terms of a geometric series with first term a₁ and common ratio r is given by the formula
[tex]S_{n}=a_{1}\cdot\dfrac{r^{n}-1}{r-1}[/tex]
Then your sum is
[tex]\sum\limits_{n=1}^{8}3^{n}=3\cdot\dfrac{3^{8}-1}{3-1}=9840[/tex]
The sum of this geometric series with first term 3 and common ratio 3 is ...
J 9840
