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

The three line segments that make up a triangle are the three sides of the triangle. The angle formed by two adjacent sides is called the interior angle of the triangle, or simply the angle of the triangle.

Vertex: represented by capital letters; Sides: Indicated by lowercase letters.

A triangle with three sides that are not equal

isosceles triangle

right triangle

obtuse triangle

Acute triangle

Draw a vertical line from a vertex of a triangle to the side opposite it. The line segment between the vertex and the vertical foot is called the height of the triangle.

The line segment connecting a vertex of a triangle to the midpoint of the side opposite it is called the midline of a triangle

The angle bisector of an angle of a triangle intersects the opposite side of the angle

(1) The three midlines are all inside the triangle - the center of gravity

(2) The three angle bisectors are all inside the triangle - the center

(3) Be sincere

(1) Theorem: The sum of the interior angles of a triangle is equal to 180°

(2) Form: In triangle ABC, ∠A ∠B ∠C=180°

(3) Proof method: cut into square angles; make parallel lines.

The symbol “Rt△

(1) Properties: The two acute angles of a right triangle are complementary to each other

(2) Decision: A triangle with two complementary angles is a right triangle

In a plane, a closed figure composed of some line segments connected end to end is called a polygon.

The angle formed by two adjacent sides of a polygon

The angle formed by a side of a polygon and the extension of its adjacent side is called the exterior angle of the polygon.

The line segment connecting two non-adjacent vertices of a polygon is called the diagonal of the polygon. [Number of entries: n(n-3)\2]

A polygon with all angles equal and all sides equal is called a regular polygon