Page 72 - Mathematics Class - IX

Unit 5 Mensura
Mensurationtion

ACTIVITY 5.1

OBJECTIVE
To form a cone from a sector of a circle and to find the formula for its curved surface area.

MATERIALS REQUIRED

Wooden hardboard Geometry box
Acrylic sheets Marker
White paper Glazed paper
Tape Nails
Scissors Marker

PRE-REQUISITE KNOWLEDGE
1. Concept of a circle
2. Concept of sector of a circle

3. Concept of a cone

THEORY
1. For concept of a circle refer to Activity 4.6.
2. Sector of the circle: The region between an arc and the two radii joining the centre to the end points of the
arc is called a sector.

Sector is of two types—minor sector and major sector. Minor sector is the sector of circle, which is less
than a semi-circle and major sector is the sector of circle, which is greater than a semi-circle (Fig. (a)).
Arclength
3. Area of sector = × Area of the circle
Circumferenceofcircle

4. Cone: A right circular cone is a solid generated by revolving a line segment which passes through a fixed
point and which makes a constant angle with a fixed line.

O Vertex
Vertex
angle
h Slant height
Major sector
l
Height Axis
O

Minor sector A r P B
A B
Radius Base
Fig. (a) Fig. (b)

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