2. The induction of the first line and the third triangle.

3. Definition and concept of the third triangle of a straight line.

4. First-line triangle Baidu Encyclopedia.

1. The conclusion of three equiangular lines is: an congruent (or similar) figure composed of three equiangular vertices on the same line. This angle can be a right angle (when a right angle is positive, there are three perpendicular lines, also called a K-shape diagram or a chord diagram), or an acute angle or an obtuse angle.

2. The third angle of a straight line is sometimes called "K-line diagram", "three perpendicular lines" and "chord line diagram". Because of the uncertainty of graphic transformation, it is often hidden in some complex graphics.