Isosceles Triangle Lesson Plan

Instructor: Mary Beth Burns

Mary Beth has taught 1st, 4th and 5th grade and has a specialist degree in Educational Leadership. She is currently an assistant principal.

In this lesson plan, students will investigate isosceles triangles. They will use an informational text to guide them in an engaging game, followed by some partner work and ending with a performance task.

Learning Objectives

By the end of the lesson, students will be able to:

  • Differentiate between isosceles triangles and other types of triangles
  • Draw an isosceles triangle using a ruler and protractor
  • Determine the measurement of the apex and second base angle in an isosceles triangle


45-60 minutes

Curriculum Standards

  • CCSS.Math.Content.7.G.A.2

Draw (freehand, with ruler and protractor, and with technology) geometric shapes with given conditions. Focus on constructing triangles from three measures of angles or sides, noticing when the conditions determine a unique triangle, more than one triangle, or no triangle.


  • Copies of What is an Isosceles Triangle? - Definition, Properties & Theorem (one per student)
  • Multiple triangles cut out of construction paper (both isosceles and non-isosceles)
  • Small empty trash can (could also use a box marked 'Trash')
  • Small empty treasure chest (could also use a box marked 'Treasure')
  • Rulers (one per student)
  • Protractors (one per student)
  • Copies of the quiz (one per student)
  • Blank paper

Lesson Instructions

What is an Isosceles Triangle? Activity

  • Students will read the section 'What is an Isosceles Triangle?' silently.
  • Have them turn and talk with a neighbor to summarize what they read.
  • Circulate the room and check for understanding.
  • Display the trash can and treasure chest. Tell the students that they are going to go on a treasure hunt for isosceles triangles in a game called 'Trash or Treasure?'.
  • Distribute rulers to students.
  • Sprinkle the triangles all over the table or floor. The goal is to be able to pick up and put away as many triangles as possible. If the triangle is isosceles, it goes in the treasure chest. If it is not isosceles, it goes in the trash can.

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