1){An} This symbol means "a series", which can be understood as the symbolic representation of the word "a series". Obviously, as a series, there must be more than one project.

2) The symbol 2)An is specially used to indicate any item in this series. It is equivalent to everyone's agreement: "Let the nth item of the series be An", which does not mean that this series has only n items. It's like setting what a letter stands for before solving an application problem. It is important to understand this article.

3) Compared with the above meaning, it is not difficult to understand that A(n+ 1) is the last term of any item in this series. For example, the series: A 1, A2, A3 …, An, A(n+ 1) …, n = 1, 2,4) In a finite series, the n representing the term can only represent the first finite natural number starting from 1; In infinite series, this n can represent any number of natural numbers.