Class 09 To verify that the sum of the angles of a triangle is 180º.

 Activity 15

  OBJECTIVE                                                                    

To verify that the sum of the angles of a triangle is 180º.

 METHOD OF CONSTRUCTION

 MATERIAL REQUIRED

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

 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, and paste it on the hardboard and name it as ΔABC.

 3.   Mark its three angles as shown in Fig. 1

 Cut out the angles respectively equal to ∠A, ∠B and ∠C from a drawing sheet using tracing paper [see Fig. 2].

5.   Draw a line on the hardboard and arrange the cut-outs of three angles at a point O as shown in Fig. 3.

 Fig. 3

 DEMONSTRATION

 The three cut-outs of the three angles A, B and C placed adjacent to each other at a point form a line forming a straight angle = 180°. It shows that sum of the three angles of a triangle is 180º. Therefore, ∠A + ∠B + ∠C = 180°.

 OBSERVATION

 Measure of ∠A = -------------------.

 Measure of ∠B = -------------------.

 Measure of ∠C = -------------------.

 Sum (∠A + ∠B + ∠C) = -------------------.

 APPLICATION

 This result may be used in a number of geometrical problems such as to find the sum of the angles of a quadrilateral, pentagon, etc