# Exterior angle property of a Triangle

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### 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)

Download this geogebra file from this link.

**Procedure:**

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

### Evaluation at the end of the activity

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

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