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

Question

mertsj · Answer

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

roadjester · Answer

Plus, you need the common ratio

anonymous · Answer

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

mertsj · Answer

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

roadjester · Answer

great, well we know it's alternating

anonymous · Answer

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

anonymous · Answer

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

anonymous · Answer

okay thank you!(: I will

roadjester · Answer

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

anonymous · Answer

Whats r?

roadjester · Answer

common ratio

anonymous · Answer

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

anonymous · Answer

how do I get r?

anonymous · Answer

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

roadjester · Answer

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

roadjester · Answer

oh, Hairt used u, I use a

anonymous · Answer

So, the 8th term is -3359232 meaning u_8=-3358232. But u_8 is also:

u_8 = u1*r^7= -3359232

anonymous · Answer

You know u_1 so you can find r

anonymous · Answer

I actually think its -3359230

roadjester · Answer

But that's only true for a geometric series.

anonymous · Answer

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

anonymous · Answer

Yeah, and this IS a geometric sequence/series

roadjester · Answer

plug in the numbers...

roadjester · Answer

a geometric sequence is $$ NOT$$ a geometric series

anonymous · Answer

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

roadjester · Answer

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

anonymous · Answer

someone could have said you use the same formula, thanks

roadjester · Answer

I thought it was self-explanatory...

roadjester · Answer

my bad

anonymous · Answer

exactly...