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.


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).


Students should already be familiar with

  • properties of parallel lines,
  • properties of similarity.


Students will not cover

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

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