help please! find Sn for the given geometric series..
a1=-10, a5=-12960, r=6
a. -15550
b.-32425
c.-11110
d. -2590

Question

help please! find Sn for the given geometric series..
a1=-10, a5=-12960, r=6
a. -15550
b.-32425
c.-11110
d. -2590

btaylor · Answer

of just the first 5 terms?

amistre64 · Answer

use the formula $$a_1\frac{1-r^n}{1-r}$$

anonymous · Answer

how do i find n amistre64?

amistre64 · Answer

n = number of terms; so there are 5 terms

anonymous · Answer

oo okay

amistre64 · Answer

a1, a2, a3, a4, a5 ... n=5 :)

anonymous · Answer

hmm well i got -7781 with that formula...which is obviously wrong...what did i do?!

amistre64 · Answer

did you keep track of your signs?

anonymous · Answer

well let me try again...

amistre64 · Answer

google calculator gives me this http://www.google.com/webhp?sourceid=chrome-instant&ie=UTF-8#hl=en&output=search&sclient=psy-ab&q=-10((1-6%5E5)%2F(1-6))&oq=&gs_l=&pbx=1&fp=3d04e930a01db91d&bav=on.2,or.r_gc.r_pw.r_qf.&biw=1280&bih=939

anonymous · Answer

..well im doing something wrong...i plugged in -10=1-6^5/1-6...i didn't include -12960..which is probably where im going wrong

amistre64 · Answer

the equal sign is not a good thing

amistre64 · Answer

-10 * (1-6^5)
------------
      (1-6)

anonymous · Answer

ooo okay multiply not equals..makes sense! thank you!

amistre64 · Answer

1-6 = -5 so -10/-5 = 2 for simplicities sake

amistre64 · Answer

2(1-6^5)

anonymous · Answer

ok i see now...thanks!

amistre64 · Answer

yw, and good luck ;)

anonymous · Answer

so wats the answer

amistre64 · Answer

@e.mccormick

hmmm, this looks familiar :)

e.mccormick · Answer

@dzenan The answer is to look at that formula, $a_1\frac{1-r^n}{1-r}$, and learn how to use it.  If you have trouble, ask your own question about how to use the formula and then you can get the answer!