2. Some geometric figures (such as cuboids, cubes, cylinders, cones, spheres, etc.). ) are not on the same plane, they are three-dimensional figures.

3. Some geometric figures (such as line segments, angles, triangles, rectangles, circles, etc.). ) are all in the same plane, which is a plane figure.

4. When the surface of the three-dimensional figure surrounded by the plane figure is properly cut, it can be expanded into a plane figure, which is called the expanded figure (net) of the corresponding three-dimensional figure.

5. Geometry is referred to as three-dimensional.

6. Surrounding the body is a curved surface, which has two kinds: plane and curved surface.

7. The intersection of surfaces forms a line, and the intersection of lines is a point.

8. Points move into faces, faces move into lines, and lines move into bodies.

9. After exploration, we can get a basic fact: there is a straight line after two points, and there is only one straight line.

Simply put, two points determine a straight line (axiom).

10, when two different straight lines have a common point, we call these two straight lines intersection, and this common point is called their intersection.

1 1, the point m divides the line segment AB into two equal line segments AM and MB, and the point m is called the center of the line segment AB.

12. After comparison, we can get a basic fact about the line segment: the line segment is the shortest of all two points. To put it simply: between two points, the line segment is the shortest. (axiom)

13, the length of the line segment connecting two points is called the distance between these two points.

14 and ∞ (angles are also basic geometric figures.

15, divide a fillet into 360 equal parts, and each equal part is an angle of 1 degree, which is recorded as1; Divide an angle of one degree into 60 equal parts, each part is called an angle of 1 minute, and it is recorded as1'; Divide the angle of 1 into 60 equal parts, and each part is called 1 sec, and it is recorded as 1 ".

16, starting from the vertex of an angle, the ray that divides this angle into two equal angles is called the angle bisector of this angle.

17, if the sum of two angles is equal to 90 (right angle), that is to say, the two angles are complementary.

Angle), that is, each angle is the complementary angle of another angle.

18, if the sum of the two angles is equal to 180 (flat angle), it is said that the two angles are complementary.

Angle), that is, one of the angles is the complement of the other.

19, the complementary angles of equal angles are equal, and the complementary angles of equal angles are equal. ?