# Lesson Plan: Parallel Lines in a Triangle Mathematics • 11th Grade

This lesson plan includes the objectives, prerequisites, and exclusions of the lesson teaching students how to find missing lengths in a triangle containing two or three parallel lines using proportionality.

#### Objectives

Students will be able to

• recognize when parallel lines inside or outside triangles create similar triangles (sometimes called the side splitter theorem),
• use proportionality to find unknown lengths in triangles containing parallel lines,
• solve multistep problems involving triangles containing parallel lines,
• prove that a straight line is parallel to one side of a triangle due to having proportional parts (the inverse of the theorem).

#### Prerequisites

Students should already be familiar with

• properties of parallel lines,
• properties of similarity.

#### Exclusions

Students will not cover

• general triangle similarity where two similar triangles are given and similarity is used to determine unknowns.