The formula a_n = a_1 + (n - 1)d is used to find the n -th term of an arithmetic sequence. In this formula, a_n represents the n -th term, a_1 is the first term, n is the term number, and d is the common difference between consecutive terms. An arithmetic sequence is a series of numbers in which the difference between any two consecutive terms is constant. For example, in the sequence 2, 5, 8, 11, the first term a_1 is 2, and the common difference d is 3. Using the formula, you can easily calculate any term in the sequence.