hkmiu
  • hkmiu
What are the first three terms of the arithmetic series if a1 = -1, an = -115, and Sn = -1160?
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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katieb
  • katieb
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anonymous
  • anonymous
isn't the formula for the sum something like \(S=\frac{n}{2}(a_1+a_n)\)?
anonymous
  • anonymous
i think that is right
anonymous
  • anonymous
so we can solve for \(n\) \[\frac{n}{2}(-1-115)=-1160\] or \[\frac{n}{2}\times (-116)=-1160\]

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More answers

anonymous
  • anonymous
divide by \(-116\) to get \(\frac{n}{2}=10\) and so \(n=20\)
anonymous
  • anonymous
Are you sure satellite? I don't remember that...
anonymous
  • anonymous
i could be wrong
anonymous
  • anonymous
No that's right.
anonymous
  • anonymous
I was thinking to find any term in an arithmetic sequence.
anonymous
  • anonymous
i could be wrong, but i am not http://en.wikipedia.org/wiki/Arithmetic_progression
anonymous
  • anonymous
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
hkmiu
  • hkmiu
Whoops i forgot to check on this.
anonymous
  • anonymous
lol you are here now right? we are not done
hkmiu
  • hkmiu
LOL, I'm here. I'm reading and catching up!
anonymous
  • anonymous
ok while you catch up i will see if i can figure out how, now that we know \(n=20\) to find \(d\)
anonymous
  • anonymous
ah yes, \(a_n=a_1+(n-1)d\)
anonymous
  • anonymous
let me know when you catch up
anonymous
  • anonymous
yeah, that's the one :P .
hkmiu
  • hkmiu
I caught up. I was abot to write the formula thn you got it :P
hkmiu
  • hkmiu
sooo its 1 + ( 20 - 1)d = an
anonymous
  • anonymous
ok so we can solve for \(d\) right?
hkmiu
  • hkmiu
Yesss! I can pick it up from here. Thank you! :)
anonymous
  • anonymous
hold on i think you have a mistake there
hkmiu
  • hkmiu
I mean a1 = -1
anonymous
  • anonymous
\[-1 + ( 20 - 1)d = -115\]
anonymous
  • anonymous
oh ok good
hkmiu
  • hkmiu
Hahahaaa ^____^ thank youuu!
anonymous
  • anonymous
yw

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