Lesson Plan: Isosceles Triangle Theorems Mathematics • 11th Grade

This lesson plan includes the objectives, prerequisites, and exclusions of the lesson teaching students how to use the isosceles triangle theorems to find missing lengths and angles in isosceles triangles.


Students will be able to

  • label the sides of an isosceles triangle as the legs and the base,
  • label the angles of an isosceles triangle as the base angles and the vertex angle,
  • prove that the base angles of an isosceles triangle are equal and use this to find missing angles,
  • prove that if two angles of a triangle are congruent, then their opposite sides are congruent and the triangle is an isosceles triangle and use this to solve problems,
  • use the isosceles triangle theorems to identify whether a triangle is isosceles.


Students should already be familiar with

  • the sum of angles in a triangle,
  • finding the area of a triangle,
  • identifying isosceles triangles,
  • equilateral triangles,
  • parallel lines and traversals.


Students will not cover

  • corollaries of the isosceles triangle theorems.

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