Angle sum property of triangles
From Open Educational Resources
The printable version is no longer supported and may have rendering errors. Please update your browser bookmarks and please use the default browser print function instead.
Source: IT for Change
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?
