Reflection: Real World Applications Triangle Inequality and Side-Angle Relationships - Section 5: Notes: Triangle Inequality, Side-Angle Relationships


Opening and shutting a door to illustrate how a triangle’s sides and angles relate to one another is not a novel idea by any means.  If we open the door just a little, we form a relatively small angle by the hinge, which produces a relatively small side opposite the angle; if we open the door wider to increase the angle by the hinge, we also increase the side opposite the angle as well.  Even though the idea seems so obvious, something about physically opening and shutting a door really helps to drive home the relationship between a triangle’s sides and angles.


One of my students shared a video that also used this door idea to help students to determine the shortest and longest sides of a triangle.  I thought this was a nice accompaniment to the discussion we had had in class for students who want to review these ideas in a different format.    

  Opening and Closing a Door to Illustrate the Relationship Between Triangle Sides and Angles
  Real World Applications: Opening and Closing a Door to Illustrate the Relationship Between Triangle Sides and Angles
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Triangle Inequality and Side-Angle Relationships

Unit 7: Discovering and Proving Triangle Properties
Lesson 3 of 10

Objective: Students will be able to explain the triangle inequality and use their understanding of the relationship between sides and angles in triangles to solve problems.

Big Idea: Through group and whole-class discussion, students will justify their reasoning with the goal of convincing others when debating the triangle inequality.

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4 teachers like this lesson
Math, Geometry, Triangle Inequality Theorem, Triangles, reasoning and proof, Triangle Congruence, triangle similarity
  85 minutes
triangle sum
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