This document discusses proportionality in triangles. It defines proportional line segments as those where the ratio of lengths on one segment equals the ratio on the other. The side-splitting theorem states that if a line parallel to one side of a triangle intersects the other two sides, it divides them proportionally and forms a similar triangle. The converse of this theorem is that if a line divides two sides of a triangle proportionally, then it is parallel to the third side. The proportional segments theorem says that if three parallel lines intersect two crossing lines, the segments formed are proportional.