Question

ACTIVITY - 11
Topic: Interior Angles of a Triangle
ans of obseravtion
class 9 activity 11 of lab manual ​

Answer 1

Answer:

Step-by-step explanation:

NCERT Class 9 Maths Lab Manual – Verify that the Sum of the Angles of a Triangle is 180°

NCERT Class 9 Maths Lab Manual – Verify that the Sum of the Angles of a Triangle is 180°

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.

NCERT Class 9 Maths Lab Manual - Verify that the Sum of the Angles of a Triangle is 180° 1

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.