Posted by **jae** on Saturday, May 26, 2012 at 5:48am.

Find the sum of the first 25 terms of an arithmetic sequence whose 7th term is −247 and whose 18th term is −49.

- arithmetic -
**Reiny**, Saturday, May 26, 2012 at 9:03am
7th term = -247 ---> a+6d = -247

18th term = -49 ---> a+17d =-49

subtract them:

11d = 198

d = 18

in a+6d = -247

a + 108 = -247

a = -355

sum(25) = (25/2)(-710 + 24(18)) = -3475

- arithmetic -
**harsh **, Saturday, March 12, 2016 at 11:38pm
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- math -
**harsh agrawal **, Saturday, March 12, 2016 at 11:43pm
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