Relation between interior and exterior angles in triangle
An exterior angle of the triangle is the angle between one side of a triangle and the extension of an adjacent side. Exterior angle of the triangle has a relation with interiors of the triangle.
Objectives
To show interior angles of a triangle have a relation with its exterior angles.
Estimated Time
40 minutes
Prerequisites/Instructions, prior preparations, if any
Prior understanding of point, lines and angles, adjacent angles, vertically opposite angles, linear pair
Materials/ Resources needed
- Digital : Computer, geogebra application, projector.
- Non digital : Worksheet and pencil.
- Geogebra files :
Download this geogebra file from this link.
Download this geogebra file from this link.
Download this geogebra file from this link.
Process (How to do the activity)
- In the triangle students should identify the angles of the triangle.
- Extend one side, students should recognize the exterior angle formed.
- What is the sum of the angles of a triangle?
- Students should be able to recognize the alternate angle formed for one of the interior angle(Angle BAC)
- Drag the parallel line to the opposite vertex, to place the alternate angle next to the angle at the opposite vertex.
- Compare the angles formed and the exterior angle, do they have a relation.
- How are the two angles together related to the exterior angle?
- Do you notice any relation between the exterior angle and the interior angles
- If you know the measure of interior angle can you find the corresponding exterior angle?
- The other two files can be used to demonstrate the the relation between the exterior angle and opposite interior angles.
- Note the measure of angles
| Triangle | Angle A | Angle B | Angle C | Exterior angle | Angle A + Angle B |
|---|---|---|---|---|---|
| Triangle1 | |||||
| Triangle2 | |||||
| Triangle3 |
Evaluation at the end of the activity
- Have the students able to identify the relation between exterior and interior opposite angles of a triangle?