Question

Find S(n) for the given arithmetic series.

a = 18, d = –10, n = 12

Answer 1

you do know you can just edit questions right?

Answer 2

I did not know that. I am noob of openstudy

Answer 3

anyway.. find a_n first using \[\huge a_n = a_1 + (n-1)d\]

Answer 4

okay one sec

Answer 5

go ahead

Answer 6

-92

Answer 7

isn't there direct formula for S_n ?

Answer 8

\[\huge S_n = \frac{n(a_1 + a_n)}{2}\]

Answer 9

you need to find a_n first

Answer 10

yeah^that

Answer 11

\(S_n=n/2(2a+(n-1)d\)

Answer 12

also..how did you get -92?

Answer 13

\(S_n=n/2(2a+(n-1)d)\)

Answer 14

well your equation. 18+(11)-10, -10*11=-110+18=-92.

Answer 15

nevermind..it's right

Answer 16

now use that very long formula

Answer 17

@Rainbow_Dash
this tutorial can be helpful to you on this topic.

Answer 18

http://openstudy.com/study#/updates/503bb2a0e4b007f9003103b0

Answer 19

thanks gohan

Answer 20

welcome :)
go through that tutorial.

Answer 21

will do.

wait, @lgbasallote , do i use your second formula with S(n)= n(a1+a_n)/2 or gohangokus?

Answer 22

they are the same

Answer 23

since u found an,u use lg's formula

Answer 24

awesome sauce. thanks guys!

Answer 25

\[\LARGE S_n = \frac{n(a_1 + a_n)}{2} \implies \frac n2 (a_1 + a_1 + (n-1)d)\]\[\LARGE \implies \frac n2 (2a_1 + (n-1)d)\]

Answer 26

it's just a fancier (and longer) way of writing it