1, the symmetry of the circle, although some other graphics also have it, but the circle has countless symmetry axes, which other graphics do not have. The vertical diameter theorem, tangent length theorem and the calculation of positive N-polygon are all applied to this characteristic.

2. A circle can be regarded as a set of points whose distance from a fixed point is equal to a fixed length.

3. Circle: The locus of a point whose distance to a fixed point is equal to a fixed length is a circle with the fixed point as the center and the fixed length as the radius.

4. Pi is a constant, representing the ratio of circumference to diameter. It is an irrational number, that is, an infinite cycle decimal.

5. circumference of a circle: the length of a curve around a circle is called the circumference of a circle, and it is represented by the letter C.