Question

What is the sum of an 8–term geometric series if the first term is 12 and the last term is –3,359,232

Answer 1

What is the formula for the sum of a geometric sequence?

Answer 2

Plus, you need the common ratio

Answer 3

I dont know, if I knew that I wouldnt be asking, like if I has a formula I could answer it myself

Answer 4

Perhaps you good google it. It isn't hard to find.

Answer 5

great, well we know it's alternating

Answer 6

\[u_1=12\]\[u_8=-3359232\]\[u_n=u_1*r^{n-1}\]\[S_n=\frac{u_1(1-r^n)}{1-r}\]Here are the formulas. What you want to find is S_8.

Answer 7

If there's anything you don't get, ask :)

Answer 8

okay thank you!(: I will

Answer 9

I've got one. What does u stand for?

Answer 10

Whats r?

Answer 11

common ratio

Answer 12

u is the term. so u_1 is the first term, which is in this case 12. and u_8 is the 8th term. r is the common ratio

Answer 13

how do I get r?

Answer 14

from the formula I gave you u_n = u1*r^(n-1)

Answer 15

r=ratio
you get the common ratio by \[\frac {a_{n+1}} a_n\]

Answer 16

oh, Hairt used u, I use a

Answer 17

So, the 8th term is -3359232 meaning u_8=-3358232. But u_8 is also:

u_8 = u1*r^7= -3359232

Answer 18

You know u_1 so you can find r

Answer 19

I actually think its -3359230

Answer 20

But that's only true for a geometric series.

Answer 21

okay so can you tell me what formula I use for the problem What is the sum of an 8–term geometric series if the first term is –11, the last term is 180,224, and the common ratio is –4

Answer 22

Yeah, and this IS a geometric sequence/series

Answer 23

plug in the numbers...

Answer 24

a geometric sequence is \[ NOT\] a geometric series

Answer 25

into what formula?? really, I not trying to be rude, but my head hurts and I am just asking for a formula not answers so please, dont tell me what I ALREADy know

Answer 26

The formula is at the top, @HairtUB gave you the formula. u1=-11, r=4, and since it's eight terms, n=8

Answer 27

someone could have said you use the same formula, thanks

Answer 28

I thought it was self-explanatory...

Answer 29

my bad

Answer 30

exactly...