beginning circle of involute profile on external flexspline

例句：

1. 1. The beginning of the story is always interesting.
   故事的开始总是有趣的。
2. 2.The new project began with a meeting last week.
   新项目上周以一次会议开始。
3. 3.The beginning of the school year is always exciting for students.
   学年开始时对学生来说总是令人兴奋的。