An arithmetic sequence is an ordered series of numbers, in which the change in numbers is constant. This method only works if your set of numbers is an arithmetic sequence.  To determine whether you have an arithmetic sequence, find the difference between the first few and the last few numbers. Ensure that the difference is always the same. For example, the series 10, 15, 20, 25, 30 is an arithmetic sequence, because the difference between each term is constant (5). Each number is a term. If there are only a few terms listed, you can count them. Otherwise, if you know the first term, last term, and common difference (the difference between each term) you can  use a formula to find the number of terms. Let this number be represented by the variable n{\displaystyle n}. For example, if you are calculating the sum of the sequence 10, 15, 20, 25, 30, n=5{\displaystyle n=5}, since there are 5 terms in the sequence. You need to know both of these numbers in order to calculate the sum of the arithmetic sequence. Often the first numbers will be 1, but not always. Let the variable a1{\displaystyle a_{1}} equal the first term in the sequence, and an{\displaystyle a_{n}} equal the last term in the sequence. For example, in the sequence 10, 15, 20, 25, 30 a1=10{\displaystyle a_{1}=10}, and an=30{\displaystyle a_{n}=30}.

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Write a summary of the previous text in Chinese:
确定数列是等差数列。 确定数列的项数。 确定数列的首项和末项。