Question

What is the sum of `5, 10, 20, 40,..., a_12`?

Answer 1

Use the geometric sum formula ...

`S_n=(a_1-a_(n+1))/(1-r)`

Explanation:

First, find `r` and `a_13` ...

`r=10/5=2`

`a_13=5(2)^((13-1))=20480`

`S_12=(a_1-a_(13))/(1-r)=(5-20480)/(1-2)=20475`

hope that helped!