## anonymous 5 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. anonymous

716,144?

2. anonymous

I got -716144

3. anonymous

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

4. anonymous

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

5. anonymous

it's same

6. anonymous

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

7. anonymous

i calculated wrongly, you are correct, sorry

8. anonymous

Haha it's ok, thank you

9. anonymous

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

10. anonymous

Ty!!! This was correct!

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