Exterior angle property of a Triangle

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To show that exterior angle of a triangle is equal to the sum of its interior opposite angles.

Estimated Time

30 minutes

Prerequisites/Instructions, prior preparations, if any

Knowledge about point, lines and angles, adjacent angles, alternate angles, corresponding angles, linear pair

Materials/ Resources needed

Digital : Click here to open the file

Non-digital : Worksheet, pencil, ruler, compass, protractor

Process (How to do the activity)

Download this geogebra file from this link.


  1. Draw triangle ABC
  2. Identify the angles of the triangle.
  3. What is the sum of the angles of a triangle?
  4. Extend one side, students should recognize the exterior angle formed.
  5. Measure the exterior angle of a triangle
  6. Identify interior opposite angles for the exterior angle of a triangle. Mark the angle in purple and green colour.
  7. Move the sliders and observe the changes.
  8. How are the two angles together related to the exterior angle?
  9. Do you notice any relation between the exterior angle and the interior angles

Evaluation at the end of the activity

  1. What would be the measure of exterior angle for each vertex of an equilateral triangle?
  2. Does an exterior angle of a triangle is smaller than either of its interior opposite angles?

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