Angle sum property of triangles

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

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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

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

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