1, line

(1) The straight line has no end point; Infinitely long; You can draw countless lines after one o'clock, and only one straight line after two o'clock.

(2) Ray has only one endpoint; Infinitely long.

(3) A line segment has two endpoints and is a part of a straight line; Limited length; In the connection between two points, the segment is the shortest.

(4) Parallel lines

On the same plane, two lines that do not intersect are called parallel lines. The vertical lines between two parallel lines are equal in length.

5] Vertical line

When two straight lines intersect at right angles, they are said to be perpendicular to each other, one of them is said to be perpendicular to the other, and the intersection point is called vertical foot.

The length of a vertical line drawn from a point outside a straight line is called the distance from that point to the straight line.

2. Angle

(1) Two rays are drawn from a point, and the graph is called an angle. This point is called the vertex of the angle, and these two rays are called the edges of the angle.

(2) Classification of angles

① Acute angle: An angle less than 90 is called an acute angle.

② Right angle: equal to

3 obtuse angle: An angle greater than 90 is called a right angle.

④ An angle of 90 and less than180 is called an obtuse angle.

⑤ Right angle: The two sides of an angle form a straight line, and the angle formed at this time is called a right angle. flat angle

Fillet: One side of the corner rotates once and coincides with the other side. Fillet 360. 180 。

plane graph

1, triangle

⑴ Features: a figure surrounded by three line segments; The sum of internal angles is 180 degrees; Triangle has stability; Draw a vertical line from the vertex of a triangle to its opposite side. The line segment between the vertex and the vertical foot is called the height of a triangle, and a triangle has three heights.

⑵ calculation formula: s=ah/2.

(3) Classification

(1) By angle.

A, acute triangle: all three angles are acute.

B. Right triangle: One angle is a right angle. The two acute angles of an isosceles triangle are 45 degrees each, and it has an axis of symmetry.

C. obtuse triangle: one angle is obtuse.

② Divide by edge

A. Unequal triangle: the lengths of three sides are not equal. B, isosceles triangle: two sides are equal in length; The two bottom angles are equal; There is an axis of symmetry.

C. equilateral triangle: all three sides are equal in length; All three internal angles are 60 degrees; There are three axes of symmetry.

2. Quadrilateral

(1) Features:

① A quadrilateral is a figure surrounded by four line segments.

② The sum of the internal angles of any quadrilateral is 360 degrees.

③ Only a set of quadrangles with parallel opposite sides is called trapezoid.

④ Two groups of parallelograms with parallel opposite sides are called parallelograms, which are easy to deform. Rectangular and square are special parallelograms; A square is a special rectangle.