A polygon with equal sides and equal angles is called a regular polygon (polygon: the number of sides is greater than or equal to 3). The center of the circumscribed circle of a regular polygon is called the center of the regular polygon. The length of the line connecting the center and the vertex of the regular polygon is called the radius. The distance between the center and the side is called the apothem. The axis of symmetry of a regular polygon - odd-numbered sides: connecting a vertex and the midpoint of the side opposite the vertex is the axis of symmetry; even-numbered sides: connecting the midpoints of two opposite sides, or connecting two symmetrical vertices, are all axes of symmetry. The number of sides of a regular N-gon is N, and the number of axes of symmetry is N.