# Angle sum property of a Triangle

### Objectives

- To establish the angle sum property of a triangle
- To help visualization of the geometric proof

### Estimated Time

30 minutes

### Prerequisites/Instructions, prior preparations, if any

Prior understanding of point, lines and angles, adjacent angles, vertically opposite angles, linear pair, parallel lines, alternate angles, corresponding angles.

### Materials/ Resources needed

Click here to open the file

### Process (How to do the activity)

Download this geogebra file from this link.

- Students should be able to recognize the corresponding angles formed when the parallel line is drawn.
- Students should be able to recognize the alternate angles formed. Is the alternate angle same as one of the angles of the triangle.
- Ask students what happens when the three angles of the triangle are placed adjacent to each other.
- What can you say about the line drawn?
- Is it parallel to one of the sides?
- What can you say about the pairs of angles – look at the matching colours.
- Once the parallel line reaches the vertex, how many angles are formed?
- Students should be able to identify the two angles moving are corresponding angles as the line moving is parallel to one of the sides.
- Students can see that when the three angles of the triangle are placed adjacent to each other they form a straight line.

### Evaluation at the end of the activity

- In a triangle, if the sum of two angles is 60°, then what will be the measure of third angle?
- If a triangle has all three angles are equal in measure, then what will the measure of each angle?

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