# 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.

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

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

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?