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.


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).


Students should already be familiar with

  • similarity,
  • solving multistep equations.


Students will not cover

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

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