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arithmetic

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

  • arithmetic -

    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 -

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  • math -

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