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

Answer 1

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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