Mathematics Lab Activity-09 Class IX | Triangles

 Lab Activity-09 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 - 09

Objective: 

To verify exterior angle property of a triangle.

Material Required

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

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/glazed paper and name it as △ABC and paste it on the hardboard, as shown in Fig. 1.

Figure 1

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

Figure 2

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

Figure 3

5. Arrange the two cutout angles as shown in Fig. 4. ∠ACD is an exterior angle.

Figure 4

6. ∠A and ∠B are its two interior opposite angles. ∠A and ∠B in Fig. 4 are adjacent angles.7. From the Fig. 4, ∠ACD = ∠A + ∠B.

Observations

1. Measure of ∠A=110^o , Measure of ∠B = 30^o,

2. Sum (∠A + ∠B) = 140^o , Measure of ∠ACD = 140^o ,

Therefore, ∠ACD = ∠A + ∠B.

Result : 

With this activity we prove that exterior angle of a triangle is equal to the sum of interior opposite angles.

Applications

This property is useful in solving many geometrical problems.

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