Question

What are the first three terms of the arithmetic series if a1 = -1, an = -115, and Sn = -1160?

Answer 1

isn't the formula for the sum something like \(S=\frac{n}{2}(a_1+a_n)\)?

Answer 2

i think that is right

Answer 3

so we can solve for \(n\)
\[\frac{n}{2}(-1-115)=-1160\] or
\[\frac{n}{2}\times (-116)=-1160\]

Answer 4

divide by \(-116\) to get \(\frac{n}{2}=10\) and so \(n=20\)

Answer 5

Are you sure satellite? I don't remember that...

Answer 6

i could be wrong

Answer 7

No that's right.

Answer 8

I was thinking to find any term in an arithmetic sequence.

Answer 9

i could be wrong, but i am not
http://en.wikipedia.org/wiki/Arithmetic_progression

Answer 10

in any case that does not answer the question, because the question is not "how many terms are there" but "what are the first 3 terms" so there is more work to do

Answer 11

Whoops i forgot to check on this.

Answer 12

lol
you are here now right?
we are not done

Answer 13

LOL, I'm here. I'm reading and catching up!

Answer 14

ok while you catch up i will see if i can figure out how, now that we know \(n=20\) to find \(d\)

Answer 15

ah yes, \(a_n=a_1+(n-1)d\)

Answer 16

let me know when you catch up

Answer 17

yeah, that's the one :P .

Answer 18

I caught up. I was abot to write the formula thn you got it :P

Answer 19

sooo its 1 + ( 20 - 1)d = an

Answer 20

ok so we can solve for \(d\) right?

Answer 21

Yesss! I can pick it up from here. Thank you! :)

Answer 22

hold on i think you have a mistake there

Answer 23

I mean a1 = -1

Answer 24

\[-1 + ( 20 - 1)d = -115\]

Answer 25

oh ok good

Answer 26

Hahahaaa ^____^ thank youuu!

Answer 27

yw