Class 07 ACTIVITY 3 - TRIANGLES

 ACTIVITY 3 - TRIANGLES

Objective: 

To verify that an exterior angle of a triangle is equal to the sum of the two interior opposite angles by paper cutting and pasting.

Materials Required: 

White sheets of paper, colour pencils, geometry box, a pair of scissors, glue stick etc.

Procedure:

On a white sheet of paper, draw a triangle ABC. Produce its side BC to D as shown in the figure. Using Colour pencils, mark its angles as shown.

2. Using a pair of scissors, cut out the two marked angular regions as shown below.

3. Paste the angular cutouts A and B over exterior angle C such that vertices A, B and C coincide as shown.

Observations:

In figure, ∠ACD is an exterior angle of ∆ABC and  ∠ A and ∠ B are its two interior opposite angles

2. In figure, we see that the angular cutouts neither overlap nor leave any gap between them. In other words, the angular cut outs A and B completely cover exterior angle C.

or ∠ A + ∠ B = exterior angle C. or ∠ A + ∠ B = ∠ ACD

Conclusion: 

From the above activity, we can say that an exterior angle of a triangle is equal to the sum of the two interior opposite angles.

Do Yourself: 

Copy each of the following triangles. In each case verify that an exterior angle of a triangle is equal to the sum of two interior opposite angles.