## anonymous one year ago What is the sum of a 7-term geometric series if the first term is 6, the last term is 24,576, and the common ratio is 4? −32,766 −19,662 19,662 32,766

1. anonymous

@jim_thompson5910

2. anonymous

i got -32, 766

3. jim_thompson5910

$\Large S_n = a*\frac{1-r^n}{1-r}$ Plug in a = 6 (first term) r = 4 (common ratio) n = 7 (number of terms)

4. anonymous

6* 1-4^7/1-4?

5. jim_thompson5910

I think you meant this right? $\Large 6*\frac{1-4^7}{1-4}$

6. anonymous

yeah!

7. jim_thompson5910

good

8. jim_thompson5910

compute that with a calculator

9. jim_thompson5910

make sure you use parenthesis

10. anonymous

i got 32766

11. jim_thompson5910

same here

12. anonymous

thanks again

13. jim_thompson5910

np

14. anonymous

will you check this one? @jim_thompson5910

15. anonymous

What is the sum of the geometric sequence 1, −6, 36, … if there are 7 terms? −39,991 −6,665 6,665 39,991 x this one?

16. jim_thompson5910

Yep $\Large S_n = a*\frac{1-r^n}{1-r}$ $\Large S_7 = 1*\frac{1-(-6)^7}{1-(-6)}$ $\Large S_7 = 39,991$