Question

Find the sum of each geometric series
3-6+12... to 7 terms

So I got S7=129 but have been told that is wrong.
My work:

S7= 3(1+2^7)/ 1+2
S7=129

Answer 1

I don't know why, I got that answer, too.

Answer 2

Well, I guess they could be wrong too. But I tried doing it manually (the pattern being multiplied by -2) and I got 192.

Answer 3

nope, 192 is the 7th term

Answer 4

S7 is sum of 7 terms, not the 7th term

Answer 5

@jim_thompson5910 Please

Answer 6

Oh, okay. So there is a difference? what would 7 terms be then?

Answer 7

you and me got S7 = 129 and you said that it's wrong, how dare I say something when I am wrong? wait for smarties.

Answer 8

Hahaha, okay. Thanks for your time.

Answer 9

it's ok, friend, I want to study , too.

Answer 10

first term: a = 3
common ratio: r = -2

Sequence is:

3, -6, 12, -24, 48, -96, 192,

so the 7th term is 192

Answer 11

and using the formula Sn = a*(1-r^n)/(1-r) gives you

Sn = a*(1-r^n)/(1-r)

S7 = 3*(1-(-2)^7)/(1-(-2))

S7 = 3*(1-(-128))/(1-(-2))

S7 = 3*(1+128)/(1+2)

S7 = 3*(129)/(3)

S7 = 129

So that confirms that the sum of the first 7 terms is 129

Answer 12

Huh, okay. So my teacher was wrong then. Thank you!

Answer 13

yeah it looks it, unless there's more to the story

Answer 14

glad to see that we are not wrong hehehe. thank you very much @jim_thompson5910

Answer 15

yw