Question

What is the sum of the geometric sequence –3, 18, –108, … if there are 8 terms?
PLEASE HELP!!

Answer 1

I'm assuming these are the first three terms in the sequence?

Answer 2

yes

Answer 3

S_8 = a_1(1-r^n)/1-r

so s_8 = -3(1- 6^8)/1-8

3/7(1-6^8)

let the scical do the rest :)

Answer 4

Scical?

Answer 5

scientific calculator

Answer 6

mcast, to clear things up a little bit, the general formula for the sum of a geometric sequence is this: \[a(1-r ^{n})\div(1-r).\]

Answer 7

it came out to be -1679615.571 when the answer choices are 719,835; -719,835; -119,973; 119,973

Answer 8

when i did the problem the answer i got was 839808 and that is still not in the choices

Answer 9

still confused

Answer 10

you there

Answer 11

"What is the sum of the geometric sequence –3, 18, –108, … if there are 8 terms?
PLEASE HELP!!" Let me see what I can do with this...(By the way, apologies to Igbasallote if i'm doing what you already did)

Let's assume this premise: \[s = a(1-r ^{n})\div(1-r)\].

Where a is the first term, s is the sum of the sequence, r is the ratio between terms, and n is the number of terms.

Our first term has a value of -3. Therefore, \[a = 3.\]
The ratio between terms, \[r\], is equal to the value of the second term divided by the first. or the third divided by the second, or the fourth divided by the third, etc, etc, etc..\[(18)\div(-3) = -6 ... r = -6.\]

Since you're aware there are 8 terms, \[n = 8.\]

Now, let's plug in to solve:\[s _{8}=(1-(-6^{8}))\div(1-(-6)).\]

\[(-6)^{8}=-1679616\]
\[(1-(-1679616)) = (1+1679616) = 1679617\]
\[(1-(-6)) = (1+6) = 7.\]
\[(1679617)\div(7)=239945.286\]
Now if i'm correct, S SHOULD...equal that value.

Answer 12

Hey...someone else should check my work, the answers he gave don't match up and i'm pretty sure that what I did was right. Something's fishy.