Judgement theorem of parallel line

Judgement theorem of parallel line

The two lines are parallel with the same angle
The internal stagger angles are equal and the two lines are parallel
The inner angles of the same side complement each other, and the two lines are parallel
If both lines are parallel to the third line, the two lines are parallel to each other

The proof of parallel line judging theorem
Two lines cut by the third line are equal, then the two lines are parallel

1. Let the intersection of the third line and two known lines be B, D; 2. You can take any point a in the third line, pass through point a, and then make any line to intersect with the two lines, and the intersection is C, E; then you can get triangle ABC and triangle ADE; 3. Prove that triangle ABC is similar to triangle ADE; 4. Triangle ABC