## anonymous one year ago Find the sum of the geometric sequence –3, 15, –75, 375, … when there are 8 terms and select the correct answer below.

1. anonymous

what is r?

2. anonymous

What is what?

3. anonymous

the common ratio

4. anonymous

Is it 5?

5. anonymous

Or -5?

6. anonymous

-5 looks right

7. SolomonZelman

$$\large\color{black}{ \displaystyle {\rm S}_{\rm n}=\frac{a_1\times \left(1-{\rm r}^{\rm n}\right)}{1-{\rm r}} }$$ this is the formula, for the sum of a geometric sequence that starts from some $$a_1$$ and has n terms.

8. SolomonZelman

you need to find r, identify the $$a_1$$ that you are given, and tell me the number of terms (which you plug in for n). plug everything into the formula, and calculate.

9. anonymous

I think I did something wrong in the equation because I got 24,414,063

10. SolomonZelman

holly molly.....

11. SolomonZelman

yeah it is wrong

12. SolomonZelman

tell me what was your common ratio (r)? what was your number of terms? what is the first term?

13. anonymous

My common ratio was -5 The number of terms was 8 The first term was -3

14. SolomonZelman

yes

15. anonymous

$$(1-(r^n))$$ is what I meant

16. SolomonZelman

$$\large\color{black}{ \displaystyle {\rm S}_8=\frac{ (-3)\times \left(1-(-5)^8\right)}{1-(-5)} }$$ Click $$\bf\color{blue }{\href{http:///www.wolframalpha.com/input/?i=%28-3+%C3%97+%281+-+%28-5%29%5E8%29%29%2F%281-%28-5%29%29}{here}}$$ to check the result.

17. anonymous

My common ratio was -5 The number of terms was 8 The first term was -3

18. anonymous

19. SolomonZelman

20. anonymous

Thank you so much!

21. SolomonZelman

anytime !