Class 09 To verify exterior angle property of a triangle.

 Activity 16

 OBJECTIVE                                                                   

To verify exterior angle property of a triangle.

 METHOD OF CONSTRUCTION

  MATERIAL REQUIRED

Hardboard sheet, adhesive, glazed papers, sketch pens/pencils, drawing sheet, geometry box, tracing paper, cutter, etc.

1.   Take a hardboard sheet of a convenient size and paste a white paper on it.

 2.   Cut out a triangle from a drawing sheet/glazed paper and name it as ΔABC and paste it on the hardboard, as shown in Fig. 1.

 Produce the side BC of the triangle to a point D as shown in Fig. 2.

 4.   Cut out the angles from the drawing sheet equal to ∠A and ∠B using a tracing paper [see Fig. 3].

 5.   Arrange the two cutout angles as shown in Fig. 4.

 Fig. 3

 Fig. 4

 DEMONSTRATION

 ∠ACD is an exterior angle.

 ∠A and ∠B are its two interior opposite angles.

 ∠A and ∠B in Fig. 4 are adjacent angles.

 From the Fig. 4, ∠ACD = ∠A + ∠B.

 OBSERVATION

 Measure    of    ∠A=  __________,  Measure  of  ∠B  =  __________,

Sum (∠A + ∠B) = ________, Measure of ∠ACD = _______.

 Therefore, ∠ACD = ∠A + ∠B.

 APPLICATION

 This property is useful in solving many geometrical problems.