Lesson plan

Classifying Triangles by Internal Angles

Classifying figures by features is a fundamental geometry skill. Use this lesson plan to teach your students how to categorize triangles by internal angle types.
Need extra help for EL students? Try theDescribing AnglesPre-lesson.
EL Adjustments
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Need extra help for EL students? Try theDescribing AnglesPre-lesson.

Students will be able to sort triangles based on features of their internal angles.

The adjustment to the whole group lesson is a modification to differentiate for children who are English learners.
EL adjustments
(5 minutes)
Sorting Triangles
  • Start off with a game of Simon Says. Direct your students to imitate you making three different angles with your arms: right angle (one at 90 degrees), an acute angle (one at less than 90 degrees), and an obtuse angle (one greater than 90 degrees).
  • As you make your angle and the students imitate you, tell them the definition of each angle you make. For example, "Simon says make a right angle; this angles is at 90 degrees."
(10 minutes)
  • Hand out and preview the Sorting Triangles worksheet and observe the shape labeled #9 in the shapes box.
  • Have students review the angle categories, turn to a neighbour and share which category triangle #9 belongs in.
  • Allow for students to determine that it is an obtuse triangle and place the number 9 on the proper section of the diagram where obtuse does not overlap with another category. Answer any clarifying questions.
(5 minutes)
  • Repeat the process for another shape, but allow students to determine the proper placement of the shape number.
  • Guide your students with leading questions as needed and answer any clarifying questions.
(15 minutes)
  • Have your students complete the remaining exercise on the Sorting Triangles worksheet.


  • Inform students to cross off items that don't qualify for categorization (e.g., the quadrilaterals) to limit potential triangle choices.
  • Students can focus on one angle type at a time (e.g., acute triangles) and cross off their choices as they go.


  • Students can make a poster for one or both of the Thinking Deeper activities using pictures, words, and numbers.
(5 minutes)
  • Show students three different types of triangles numbered 1, 2, and 3. Say a category (i.e., obtuse triangle, acute triangle, right triangle) and have them show a number with their fingers for the category the triangle belongs to.
  • Choose students to share their rationale using some of the key terms from the lesson.
(10 minutes)
  • Review the answers as a class and allow for peers to assist with incorrect responses.
  • Assign an exit ticket activity where students modify one of their choice of angles 6–8 so that they COULD be placed in one of the three diagram categories.

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