# Lesson Plan: Parallel Lines and Transversals: Proportional Parts Mathematics • 11th Grade

This lesson plan includes the objectives, prerequisites, and exclusions of the lesson teaching students how to use parallelism of lines to find a missing length of a line segment in a transversal line cut by parallel lines.

#### Objectives

Students will be able to

• understand how to apply the basic proportionality theorem (Thales’s theorem) to find a missing length of a line segment in a transverse line cut by parallel lines
• including problems involving algebraic variables as side lengths,
• including problems that use the properties of polygons,
• understand that if the lengths of the segments of one transversal are equal, then the lengths of the segments of any other transversal are also equal (Thales’s special theorem).

#### Prerequisites

Students should already be familiar with

• similarity,
• solving multistep equations.

#### Exclusions

Students will not cover

• trigonometry,
• the sine and cosine rule,
• parallel lines in a triangle.