2. Any line segment can be extended into a straight line.

3. You can make a circle with a point as the center and a line segment as the radius.

All right angles are equal.

5. There is one and only one straight line parallel to a point outside the known straight line.

Axiom is a hypothesis and cannot be proved. Starting from these five axioms, Euclid deduced a series of theorems.

Extended data:

A straight line consists of countless points. A straight line is a part of a surface and then constitutes a body. There is no end point, it extends infinitely to both ends, and the length cannot be measured. You can draw countless straight lines through a point. You can draw a straight line through two points.

A straight line is the basic concept of geometry, which is the trajectory of a point in space moving in the same or opposite direction. Or defined as: curve with minimum curvature (arc with infinite radius). A straight line has countless symmetry axes, one of which is itself, and all straight lines perpendicular to it (there are countless) symmetry axes. 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 direction of a straight line in space is represented by a non-zero vector parallel to the straight line, which is called the direction vector of the straight line. The position of a straight line in space is completely determined by a point and a direction vector in the space it passes through. In Euclidean geometry, straight lines are only intuitive geometric objects. When establishing Euclidean geometry axiom system, lines, points, planes, etc. Is undefined, and their relationship is described by a given axiom.