Angle sum property

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Interior angles of a triangle are in relation and also determine the type of angles that can forms a triangle. This also helps in determining an unknown angle measurement.

Objectives

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

Estimated Time

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

Process (How to do the activity)


Download this geogebra file from this link.


  • Use the file - “a.Angles in a right triangle.ggb
  • Ask students what is the kind of triangle they observe.
  • Draw a parallel line to the side containing the right angle through the opposite vertex by dragging point D along the x-axis
  • 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.
  • So what can you say about the all the angles of the triangle?


Download this geogebra file from this link.


  • With the file - “b. Angle sum property proof.ggb
  • Draw a line perpendicular to a side passing through the opposite vertex. How many triangles are formed. What kinds of triangles are formed?
  • In each of the two triangles if on angle is 90o, what will be the sum of the other two angles. What is the sum of these angles?
  • Children can record the values of the angles of a triangle in the worksheet
Observation Angle 1 Angle 2 Angle 3 Angle 1 + Angle 2 + Angle 3 What can you say about sum of angles?
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Download this geogebra file from this link.


  • With the file – “c. Angle sum property of a triangle.ggb
  • 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 colors.
  • 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

  • Have students able to conclude if the sum of angles in any triangle is 180o?