factorial series

例句：

1. 1. The math teacher explained the concept of factorials to the class.
   数学老师向班上解释了因数的概念。
2. 2.In factorial series, each term is the product of all the preceding terms.
   在因数序列中，每个项都是前一项的乘积。
3. 3. The factorial of a number n! is the product of all positive integers less than or equal to n.
   一个数n的阶乘是所有小于或等于n的正整数的乘积。