1. rotation: in a plane, a graph rotates an angle around a graph in a certain direction, which is called graph rotation. This fixed point is called the center of rotation and the rotation angle is called the rotation angle. The rotation of a graph is that every point on the graph rotates around a fixed point at a fixed angle on the plane. Position movement, in which the distance from the corresponding point to the center of rotation is equal, the length of the corresponding line segment is equal to the size of the corresponding angle, and the size and shape of the figure have not changed before and after rotation. )

1. Rotational symmetry center: a figure rotates around a fixed point by an angle and then coincides with the original figure. This figure is called rotationally symmetric figure, this fixed point is called rotationally symmetric center, and the rotation angle is called rotation angle (rotation angle is less than 0 and greater than 360).

3. Centrally symmetric figures and centrosymmetry

Centrally symmetric figure: If a figure can overlap itself after rotating 180 degrees around a certain point, then we say that this figure has formed a centrosymmetric figure.

Central symmetry: If one graph can overlap another graph after rotating 180 degrees around a certain point, then we say that these two graphs form central symmetry.

4. The essence of central symmetry

On the congruence of two graphs with central symmetry.

For two graphs with central symmetry, the straight lines connecting the symmetrical points pass through and are equally divided by the symmetrical center.

For two figures with symmetrical centers, the corresponding line segments are parallel (or on the same straight line) and equal.