Exterior angle property of a Triangle

Objectives
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)
Procedure:
 * 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
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 * 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?