Interior and exterior angles in triangle

Objectives

 * Identify all angles when a triangle is formed
 * Understand the relation between various angles that are formed in a triangle.

Prerequisites/Instructions, prior preparations, if any
Prior knowledge of point, lines, angles

Materials/ Resources needed

 * Digital : Computer, geogebra application, projector.
 * Non digital : Worksheet and pencil.
 * Geogebra files : “Angles of  triangle.ggb”

Process (How to do the activity)

 * Ask students how many lines are there? They should be able to identify the points of intersection of the lines. How many points of  intersection are formed?
 * How many angles are formed at an intersecting point? How many angles in total at the three points of intersection?What is the total  angle measure at each intersecting point?
 * How many angles are inside the triangle and how many are outside the triangle
 * Can you find an exterior angle that is equal to the interior angle of a triangle at each vertex?Why are they equal?
 * Identify the exterior angles that are equal? Justify why they are equal.
 * Establish that there are 2 angles which are exterior of the triangle that are equal and are formed when the sides of the triangle is  extended at the vertex.
 * Students to analyze the interior and exterior angle at each point to find a relation between the interior angle and one of the exterior angles at the vertex. Students should be able to recognize the linear pair formed by interior angle and exterior angle.
 * Vary the position of the lines to check if interior and exterior angles form a linear pair.

Evaluation at the end of the activity
 * Note the measure of angles
 * Are students able to recognize interior and exterior angles in a  triangle
 * Have the students able to find a relation between the interior angle and  exterior angle that are formed at each vertex?