Exterior angle property of a triangle

Procedure to do activity in the simulation:

Steps to prove the property using this lab are as follows

A. Steps to draw a triangle

  1. Go to the TOOLS and select the triangle.
  2. Mark a point A on baseline to draw a triangle ABC.
  3. Click again on baseline for another point B.
  4. Click above baseline on the benchmark to mark the point C.

Triangle drawn successfully.

B. Steps to mark the interior angles and exterior angle

  1. Click on point A to mark ∠CAB
  2. Click on point B to mark ∠ABC,    
  3. Click on point C to mark ∠ACB,      
  4. Click on point D to mark ∠CBD(exterior angle) of triangle ABC.     

C. Measure the values of resultant angles 

  1. Drag the protractor to measure the value of the interior angles,  ∠CAB and ∠ACB.     
  2. Consider AD as a baseline for ∠CAB and line CB for ∠ACB. 
  3. Enter the value of angles ∠CAB and ∠ACB in text box. 
  4. Measure the value of angle ∠CBD i.e. exterior angle from base line AD.
  5. Enter the value in text box.    

D. Verify the theorem

  1. Let us cut and place the interior angles on exterior angle
  2. Click on point A  to cut angle ∠CAB.      
  3. Click on point  C  to cut angle ∠ACB.      


  1.     Click on Next button to see the observation.   

We observe that the interior angles completely overlap the exterior angle.

We also observe that ∠CAB +∠ACB  = ∠CBD 

Thus, The sum of value of interior angles ∠CAB  and ∠ACB   is equal to the exterior angle ∠CBD.

Hence exterior angle property of triangle is proved.