This document discusses proportionality theorems that can be used to calculate unknown segment lengths in triangles. It introduces the Triangle Proportionality Theorem and its converse, which state that if a line parallel to one side of a triangle intersects the other two sides, it divides them proportionally. An example problem demonstrates using these theorems to find the length of segment SU given other segment lengths and that TU is parallel to QS. Additional proportional theorems and practice problems are also presented.