The document discusses arithmetic sequences and series. An arithmetic sequence is a sequence where the difference between successive terms is constant. The nth term can be calculated using the formula an = a1 + (n - 1)d, where a1 is the first term, d is the common difference, and n is the term number. The sum of an arithmetic sequence is called an arithmetic series, which can be calculated using the formula Sn = (n/2)(a1 + an), where Sn is the sum, n is the number of terms, a1 is the first term, and an is the last term.