1. triangle: A figure composed of three line segments that are not on the same line and are connected end to end is called a triangle.

2. Trilateral relationship: the sum of any two sides of a triangle is greater than the third side, and the difference between any two sides is less than the third side.

3. Height: Draw a vertical line from the vertex of the triangle to the line where the opposite side is located, and the line segment between the vertex and the vertical foot is called the height of the triangle.

4. midline: in a triangle, the line segment connecting the vertex and its relative midpoint is called the midline of the triangle.

5. Angular bisector: The bisector of the inner angle of a triangle intersects the opposite side of this angle, and the line segment between the intersection of the vertex and this angle is called the angular bisector of the triangle.

6. Stability of triangle: The shape of triangle is fixed, and this property of triangle is called stability of triangle.

6. Polygon: On the plane, a figure composed of some line segments connected end to end is called polygon.

7. Interior Angle of Polygon: The angle formed by two adjacent sides of a polygon is called its interior angle.

8. Exterior angle of polygon: The angle formed by the extension line of one side of polygon and its adjacent side is called the exterior angle of polygon.

9. Diagonal line of polygon: The line segment connecting two non-adjacent vertices is called diagonal line of polygon.

10. Regular polygon: A polygon with equal angles and sides in a plane is called a regular polygon.

1 1. plane mosaic: covering a part of a plane with some non-overlapping polygons is called covering the plane with polygons.

12. Formula and properties: the sum of the internal angles of a triangle is 180. The nature of the exterior angle of a triangle: nature.

1: One outer angle of a triangle is equal to the sum of two inner angles that are not adjacent to it.

2: The outer angle of a triangle is greater than any inner angle that is not adjacent to it.

The formula for the sum of the internal angles of polygons: the sum of the internal angles of n polygons is equal to the sum of the external angles of (n-2) 180; the sum of the internal angles of polygons is 360. Number of diagonal lines of polygon:

(1) Starting from a vertex of an n polygon, (n-3) diagonal lines can be drawn, and the polygon can be divided into (n-2) triangles.

(2)n sides * * * have 23)-n(n diagonal lines.