Formation of a triangle

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Introducing formation of a shape with least number of lines and the space enclosed by these lines form a geometric shape.The key geometric concepts that are related with this are explained.


  • Understand formation of triangles
  • Recognize elements of triangle
  • Introduce concepts of exterior angle.

Estimated Time

30 minutes

Prerequisites/Instructions, prior preparations, if any

Prior knowledge of point, lines, angles, parallel lines

Materials/ Resources needed

Download this geogebra file from this link.

Process (How to do the activity)

  • Use the geogebra file to illustrate. The questions below will are used to interact with the geogebra sketch.
  • How many lines are there? Are the lines meeting?
  • Are the two lines parallel? How can you say they are parallel or not?
  • How many angles are formed at the point of intersection?
  • What is the measure of the total angle at the point of intersection of two lines?
  • Of the four angles formed which of the angles are equal? What are they called?
  • Do the three intersecting lines enclose a space? How does it look? It is called a triangle.
  • What are the points of intersection of these three lines called?
  • The line segments forming the triangle are called sides.
  • How many angles are formed when three lines intersect with each other?
  • How many angles are enclosed by the triangle?

Evaluation at the end of the activity

  • Can there be a closed figure with less than three sides?
  • Can the vertices of the triangle be anywhere on a plane?
  • What will happen if the three vertices are collinear?