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1, straight line
A straight line consists of countless points, and the points move into a line. A straight line is a part of a surface and then constitutes a body. There is no end point, extending to both ends indefinitely, and the length cannot be measured. A straight line is an axisymmetric figure.
It has countless symmetry axes, all of which are straight lines perpendicular to it (there are countless). There is only one straight line between two non-overlapping points on the plane, that is, two non-overlapping points determine a straight line. On the sphere, countless similar straight lines can be made after two points.
The most basic element of geometry. In the axiomatic system of Euclidean geometry established by D Hilbert, points, lines and surfaces belong to basic concepts, which are defined by their relationships and five groups of axioms.
Step 2 light
Ray refers to a straight line formed by the infinite extension of one end of a line segment. A ray has only one endpoint, so it is impossible to measure its length (it is infinitely long).
In Euclidean geometry, the figure formed by a point on a straight line and its edge is called a ray, and this point is called the endpoint of the ray.
For the light in geometry, we usually visually regard it as the light emitted by a flashlight.
If the endpoint is A and any point on the ray except the endpoint is B, then this ray can be recorded as ray AB.
Note: endpoint a is in front, and another point b is behind. Otherwise something will go wrong.
Two rays with the same endpoint and different directions are two different rays.
Two rays with the same endpoint and the same direction are the same ray.
3. Line segment
A line segment is a limited part (including two endpoints) between two points on a straight line, which is different from a straight line and a ray.
Line segment, a general term in technical drawing, refers to a continuous or discontinuous drawing line composed of one or more different line elements, such as a real line segment or a two-point long line segment composed of "long stroke, short interval, point, short interval, point and short interval".
Connect two points with a ruler and you get a line segment. The length of a line segment is the distance between these two points.
The length of the line segment connecting two points is called the distance between these two points.
Line segments are represented by letters A and B or lowercase letters representing their two endpoints. Sometimes these letters also represent the length of the line segment, which is recorded as line segment AB or line segment BA, line segment A .. where A and B represent the two endpoints of the line segment.
Generally speaking, a line segment is composed of countless points, which is often said in textbooks.
We think this statement is correct. In fact, this problem has been studied by many individuals.
After deliberation and debate by people from all walks of life, the following questions are raised: If a line segment is composed of points, is it finite or infinite? If it is a finite number, do these points have a length? If it is infinite, is there a gap between these points?
If there is no space between points, then points cannot be said to have length, that is, they are all isolated, and the length of line segments cannot be obtained; If there is a gap between points, can you insert another point between two separated points? If there are gaps, how many points can be inserted between them?
The correct statement is that a line segment consists of an infinite number of points, and the length of the line segment has nothing to do with the length of the points. Two values with different scales cannot be simply extrapolated directly. Finite and infinite cases cannot be simply extrapolated. Detailed discussion is the content of higher mathematics.
There is also a saying that is explained from the viewpoint of motion: a line segment is the locus of a point. But in real life, people have long acquiesced in the saying that "line segments are composed of countless points".