## mitosuki 3 years ago What is the sum of an 8–term geometric series if the first term is –11, the last term is 859,375, and the common ratio is –5? –143,231 716,144 144,177 –36,047

1. mitosuki

716,144?

2. Tomas.A

I got -716144

3. Tomas.A

what formula you used? $S=a\frac{1-r^n}{1-r}$ this?

4. mitosuki

No it looks like: a1 - a1 (r)^n Sn = _______________ 1 - r

5. Tomas.A

it's same

6. Tomas.A

a1-a1r^n=a1(1-r^n)

7. Tomas.A

i calculated wrongly, you are correct, sorry

8. mitosuki

Haha it's ok, thank you

9. Tomas.A

i calculated as $S=a\frac{r^n-1}{1-r}$ mixed r^n and 1 ><