Angular shape: A figure composed of three line segments that are not on the same straight line is called triangle edge 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 height of the third side; a vertical line is drawn from a vertex of a triangle to the straight line where its opposite side is located, and the line segment between the vertex and the vertical foot is called a triangle.

Midline: In a triangle, the line segment connecting the vertex and the midpoint of its opposite side is called the bisector of the triangle: the bisector of an inner corner of the triangle intersects the opposite side of the corner, and the line segment between the intersection of the vertex and the corner is called the bisector of the triangle.

Stability of triangle: the shape of triangle is fixed, and this property of triangle is called the stable polygon of triangle: on the plane, the figure composed of some end-to-end line segments is called the inner angle of polygon; The angle formed by two adjacent sides of a polygon is called its inner angle.

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

Introduction to mathematics:

Mathematics [English: Mathematics, from ancient Greece μ? θξμα(máthēma)； Often abbreviated as math or maths], it is a discipline that studies concepts such as quantity, structure, change, space and information.

Mathematics is a universal means for human beings to strictly describe and deduce the abstract structure and mode of things, and can be applied to any problem in the real world. All mathematical objects are artificially defined in essence. In this sense, mathematics belongs to formal science, not natural science.

Related introduction:

Different mathematicians and philosophers have a series of views on the exact scope and definition of mathematics. Mathematics plays an irreplaceable role in the development of human history and social life, and it is also an indispensable basic tool for studying and studying modern science and technology.