(1) line

Straight line: straight line has no end; Infinitely long; You can draw countless lines after one o'clock, and only one straight line after two o'clock.

Ray: Ray has only one endpoint; Infinitely long.

Line segment: 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.

Parallel lines: In the same plane, two disjoint lines are called parallel lines.

The vertical lines between two parallel lines are equal in length.

Verticality: 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 drawn from a point form a graph 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

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

Right angle: An angle equal to 90 is called a right angle.

Oblique angle: an angle greater than 90 and less than180 is called obtuse angle.

Flat angle: The two sides of an angle form a straight line, and the angle formed at this time is called a flat angle. Boxer 180.

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

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