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