There is only one straight line between two points.

Simply put: two points determine a straight line.

2

The line segment between two points is the shortest.

three

The complementary angles of the same angle or the same angle are equal.

four

The complementary angles of the same angle or the same angle are equal.

five

Equal vertex angle

six

One and only one straight line is perpendicular to the known straight line.

seven

Of all the line segments connecting the outer point and the point on the line, the vertical line segment is the shortest.

Eight parallel axioms

After passing a point outside the straight line, there is one and only one straight line parallel to this straight line.

If both lines are parallel to the third line, the two lines are also parallel to each other.

10

The isosceles angles are equal and the two straight lines are parallel.

1 1 The internal dislocation angles are equal and the two lines are parallel.

12

The internal angles on the same side are complementary and the two straight lines are parallel.

13 Two straight lines are parallel and have the same angle.

14

Two straight lines are parallel and have equal internal angles.

15

These two lines are parallel and complementary.

16

theorem

The sum of two sides of a triangle is greater than the third side.

17

reason

The difference between two sides of a triangle is smaller than the third side.

The theorem of sum of interior angles of 18 triangle

The sum of the three internal angles of a triangle is equal to 180.

19 inference 1

The two acute angles of a right triangle are complementary.

20 Inference 2

The outer angle of a triangle is equal to the sum of two non-adjacent inner angles.

2 1

Inference 3

The outer angle of a triangle is greater than any inner angle that is not adjacent to it.

22

The sum of the outer angles of a polygon is equal to 360 degrees.