Straight line; A straight line is infinite.
Line segment: A segment between two points on a straight line is called a line segment. A line segment has two endpoints. A line segment is a part of a straight line.
Ray: Extend one end of a line infinitely, and you get a ray. A ray has only one endpoint.
Angle: The figure formed by two rays from a point is called an angle. This point is called the vertex of the angle. These two rays are called the edges of the angle. The angle is usually represented by the symbol "∞". As shown in the figure below:
edge
pinnacle
edge
Compare the sizes of angles: first overlap the vertices of two angles on one side, and then look at the position of the other side. Which corner has the other side outside, which corner is big. If the other side also coincides, the two angles are equal.
The size of the angle depends on the size of both sides. The bigger the fork, the bigger the angle. The size of the angle has nothing to do with the length drawn on both sides of the angle.
Measurement of angle: the unit of measurement of angle is "degree", which is represented by the symbol "0". Divide the semicircle into 180 equal parts, and the angle of each part is called 1 degree angle. Write 1. When measuring an angle with a protractor, place the protractor above the angle so that the center of the protractor coincides with the vertex of the angle. The 0 degree line coincides with one side of the angle, and the scale on the protractor on the other side of the angle is the degree of this angle.
Classification of angles: angles greater than 0 and less than 90 are called acute angles. An angle equal to 90 degrees is called a right angle. An angle greater than 90 and less than180 is called an obtuse angle. The two sides of an angle form a straight line, and the angle equal to 180 is called a right angle. The 360-degree angle formed by light rotating around its endpoint is called fillet.
Vertical line: When two lines intersect at right angles, they are called perpendicular to each other, one of which is called the vertical line of the other (as shown in the following figure 1), and the intersection of these two lines is called vertical foot.
Parallelism: Two straight lines that never intersect in the same plane are called parallel lines (Figure 2 below). It can also be said that these two straight lines are parallel to each other.