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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