Final answer:
An arithmetic sequence is defined by the formula an = a1 + (n - 1)d, where an is the n-th term, a1 is the first term, n is the term number, and d is the common difference.
Step-by-step explanation:
An arithmetic sequence is a sequence of numbers in which each term after the first is found by adding a constant, known as the common difference, to the previous term. The standard form for writing an arithmetic sequence is an = a1 + (n - 1)d, where an is the n-th term of the sequence, a1 is the first term, n is the term number, and d is the common difference.
For example, if we are given the sequence 3, 5, 7, 9, the first term a1 is 3 and the common difference d is 2 (since each term is increased by 2). The standard form of this sequence is an = 3 + (n - 1)×2.