Question

Find S8 for the geometric series 3 + -6 + 12 + -24 +…
A. -384
B. 192
C. -96
D. -255

Answer 1

\[S_n = a \frac{ 1 - r^n }{ 1 - r }\]

'a' represents the first term you have (3)
'n' represents the amount of numbers you want to sum (8)
'r' represents the common ratio....(?)

what is your common ratio?

Answer 2

n=8 ?
a = -3
what does r = ?

Answer 3

right n = 8
a = 3 not -3 right?

r...what do we multiply 3 by..to get -6?
and what do we multiply -6 by....to get 12?

Answer 4

-2

Answer 5

perfect

so put everything together knowing
a = 3
r = -2
and n = 8

\[S_8 = 3\frac{ 1 + 2^8 }{ 1 + 2 }\]

Answer 6

Wait a minute...is this finding a sum? or finding the 8th term??

Answer 7

i beilve the sum

Answer 8

It must be finding the 8th term....because the equation doesn't work for me

and when finding the 8th term...I get -384 which is one of the answers. let me recheck real quick

Answer 9

nvm wrote it wrong..ignore the above equation post

Answer 10

\[S_8 = 3\frac{ 1 - (-2^8) }{ 1 + 2 }\]

forgot those parenthesis and the fact that -2 gets the exponent first

Answer 11

and this does indeed give me one of those answers...sorry about that confusion

Answer 12

And once you do that CORRECT equation *again sorry about the confusion* you should get -255