Mathematics Lab Activity-08 Class IX | Triangles

   Lab Activity-08 Class iX

Mathematics Lab Activities on Triangles for class IX students, with complete observation tables, strictly according to the CBSE syllabus.

Chapter - 07  Triangles

Activity - 08

Objective: 

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

Material Required

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

Procedure

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 △ABC.

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

Figure 1

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

Figure 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.

Figure 3

6. 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°. 

7. It shows that sum of the three angles of a triangle is 180º. Therefore, ∠A + ∠B + ∠C = 180°.

Observations

Measure of ∠A = 110°.
Measure of ∠B = 30°.
Measure of ∠C = 40°.
Sum (∠A + ∠B + ∠C) = 180°.

Result : 

From this theorem we prove that sum of three angles of triangle is always = 180°.

Applications

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

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