Understanding:
The sum of a geometric series is, [tex]a+ar+ar^2+ar^3+...[/tex], with a being the start point and r is the common ratio. We can also use the following formula to make life easier [tex]S_n=a\frac{1-r^n}{1-r}[/tex], a is your start point, r is the common ratio, and n is the number of terms, which in our case is S7.
Solution:
Our start point is 1, [tex]a=1[/tex],
The common ratio is 5, [tex]r=5[/tex],
And finally, the number of terms is 7, [tex]n=7[/tex].
[tex]S_n=a\frac{1-r^n}{1-r} \\=(1)\frac{1-(5)^{(7)}}{1-(5)} \\=\frac{1-78125}{1-5} \\=\frac{-78124}{-4}\\=19531[/tex]
The answer is [A] 19531.
