1, the definition of a straight line
A straight line is the most basic element of a geometric figure and consists of countless points. A straight line has no end points and extends infinitely to both ends, and its length cannot be measured. A straight line is a part of a surface and then constitutes a body. A straight line has countless symmetry axes, one of which is the straight line itself, and all straight lines perpendicular to the straight line are symmetry axes.
On the plane, there is only one straight line between two points passing through non-coincidence. On the sphere, countless similar straight lines can be made after two points. Generally speaking, the distance from a point to a straight line is the shortest distance from a point to a straight line, which is the vertical distance. No matter whether they coincide or not, two intersecting straight lines can intersect or be parallel on the two-dimensional plane.
2. Definition of ray
Ray refers to a straight line formed by the infinite extension of one end of a line segment. The ray has only one endpoint, so the length cannot be measured. In Euclidean geometry, a figure consisting of a point and an edge on a straight line is called a ray. In geometry, light is usually visually regarded as the light emitted by a flashlight.
3. Definition of line segment
A line segment means that both ends have endpoints and cannot be extended, which is different from straight lines and rays.
Representation of lines, rays and line segments
1, the representation of the line segment
First name the two endpoints of the line segment with capital letters, one endpoint is A and the other endpoint is B, then the specified line segment is AB. There is a knowledge point, and two points determine a line segment.
2. Representation of straight lines
First name two points on a straight line with capital letters, one is A and the other is B, then the designated straight line is AB. There is a knowledge point, and two points determine a straight line.
3, ray representation
A ray can be represented by an endpoint and another point on the ray. The letter of the end point (starting point) is written in front and cannot be changed. The endpoint of the ray is a, and the other point is b, which is ray ab; The endpoint of the ray is b, and the other point is a, that is the ray BA.