Question

Find the sum of the geometric sequence –3, 15, –75, 375, … when there are 8 terms and select the correct answer below.

Answer 1

what is r?

Answer 2

What is what?

Answer 3

the common ratio

Answer 4

Is it 5?

Answer 5

Or -5?

Answer 6

-5 looks right

Answer 7

\(\large\color{black}{ \displaystyle {\rm S}_{\rm n}=\frac{a_1\times \left(1-{\rm r}^{\rm n}\right)}{1-{\rm r}} }\)

this is the formula, for the sum of a geometric sequence that starts from some \(a_1\) and has n terms.

Answer 8

you need to find r, identify the \(a_1\) that you are given, and tell me the number of terms (which you plug in for n).

plug everything into the formula, and calculate.

Answer 9

I think I did something wrong in the equation because I got 24,414,063

Answer 10

holly molly.....

Answer 11

yeah it is wrong

Answer 12

tell me what was your common ratio (r)?
what was your number of terms?
what is the first term?

Answer 13

My common ratio was -5
The number of terms was 8
The first term was -3

Answer 14

yes

Answer 15

\((1-(r^n))\) is what I meant

Answer 16

\(\large\color{black}{ \displaystyle {\rm S}_8=\frac{ (-3)\times \left(1-(-5)^8\right)}{1-(-5)} }\)

Click \(\bf\color{blue }{\href{http:///www.wolframalpha.com/input/?i=%28-3+%C3%97+%281+-+%28-5%29%5E8%29%29%2F%281-%28-5%29%29}{here}}\) to check the result.

Answer 17

My common ratio was -5
The number of terms was 8
The first term was -3

Answer 18

Is the answer 195,312??

Answer 19

yes, that is the answer.

Answer 20

Thank you so much!

Answer 21

anytime !