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The 13th termof an arithmetic sequence is 61 and the 20th term of the squence is 82. Calculate the sum of the first 25 terms of the sequence

  • Math -

    T13 = a+12d = 61
    T20 = a+19d = 82
    so,
    T20-T13 = 7d = 21
    d = 3
    a = 25

    S25 = 25/2 (25 + 25+24*3)
    = 25/2 (122) = 25*61 = 1525

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