1. Definition of a circle: The textbook introduces the definition of a circle, that is, the set of points with equal distance from the center of the circle.
2. Components of a circle: The textbook introduces the components of a circle, including the center, radius and diameter.
3. The nature of the circle: The textbook introduces some basic properties of the circle. For example, the diameter of the circle is the longest line segment on the circle, and the line segment between any two points on the circle does not exceed the diameter of the circle.