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Lab Manual: Verify that the Sum of the Angles of a Triangle is 180° | Lab Manuals for Class 9 PDF Download

Objective

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

Materials Required
Cardboard sheet, White chart paper, Geometry box, Scissors, Adhesive, Drawing sheet, Tracing paper

Prerequisite Knowledge

  • Knowledge of straight angle
  • Concept of triangle and its properties

Theory

  • For straight line angle refer to Activity 11.
  • Triangle
    A plane figure closed by three intersecting lines is called a triangle, here Tri’ means ‘three’. A triangle has three sides, three angles and three vertices and it is denoted by the symbol ‘Δ’. A ΔABC is shown in Fig. 12.1.Lab Manual: Verify that the Sum of the Angles of a Triangle is 180° | Lab Manuals for Class 9

Some basic properties of triangles are given below:

  • The sum of all the angles of a triangle is always 180°.
  • Angles opposite to equal sides of a triangle are equal.
  • The sides opposite to equal angles in a triangle are equal.
  • In a triangle, the angle opposite to longer side is larger and vice-versa.
  • The sum of two sides of a triangle is greater than the third side.
  • The exterior opposite angle in a triangle is equal to sum of opposite interior angles.

Note: The property (1) is called Angle Sum Property of triangle.

Procedure

  • Take a cardboard of suitable size and by using adhesive, paste a white chart paper on it.
  • Draw a ΔABC of suitable lengths on the drawing sheet and also mark all the angles as shown in Fig. 12.2.
    Lab Manual: Verify that the Sum of the Angles of a Triangle is 180° | Lab Manuals for Class 9
  • By using tracing paper, cut out the angles respectively equal to ∠A , ∠B and ∠C from a drawing sheet, (see Fig. 12.3).
    Lab Manual: Verify that the Sum of the Angles of a Triangle is 180° | Lab Manuals for Class 9
  • Now, draw a line on the cardboard and paste the cut outs of the angles (∠A, ∠B and ∠C) on the line at a point O as shown in Fig. 12.4.
    Lab Manual: Verify that the Sum of the Angles of a Triangle is 180° | Lab Manuals for Class 9

Demonstration
When all the three cut outs of the angles A, B, C placed adjacent to each other at a point, then it forms a line forming a straight angle, i.e. 180°.
Flence, it is proved that the sum of the three angles of a triangle is 180°.
Therefore, ∠A + ∠B + ∠C =180°

Observation
Measure of ∠A = ………… ,
Measure of ∠B = ………… ,
Measure of ∠C = ………… ,
Hence, sum (∠A + ∠B + ∠C) = ………… .

Result
The sum of angles of a triangle is 180°, have been verified.

Application
This result can be used in many geometrical problems such as to find the sum of angles of a quadrilateral, pentagon and hexagon etc.

The document Lab Manual: Verify that the Sum of the Angles of a Triangle is 180° | Lab Manuals for Class 9 is a part of the Class 9 Course Lab Manuals for Class 9.
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