NCERT Class 9 Mathematics

216 MATHEMATICS

of ✁ is a non-terminating, non-repeating decimal. But when we use its value in our

calculations, we usually take its value as approximately equal to

22

7

or 3.14.

Example 3 : Savitri had to make a model of a cylindrical kaleidoscope for her science
project. She wanted to use chart paper to make the curved surface of the kaleidoscope.
(see Fig 13.10). What would be the area of chart paper required by her, if she wanted

to make a kaleidoscope of length 25 cm with a 3.5 cm radius? You may take ✁ =

22

7

.

Solution : Radius of the base of the cylindrical kaleidoscope (r) = 3.5 cm.

Height (length) of kaleidoscope (h) = 25 cm.

Area of chart paper required = curved surface area of the kaleidoscope

=2✁rh

=^2

22

23 .525cm

7

= 550 cm^2

EXERCISE 13.2

Assume ✂=^22

7

, unless stated otherwise.

1. The curved surface area of a right circular cylinder of height 14 cm is 88 cm^2. Find the
   diameter of the base of the cylinder.


2. It is required to make a closed cylindrical tank of height 1 m and base diameter 140 cm
   from a metal sheet. How many square metres of the sheet are required for the same?


3. A metal pipe is 77 cm long. The inner diameter of a cross
   section is 4 cm, the outer diameter being 4.4 cm
   (see Fig. 13.11). Find its
   (i) inner curved surface area,
   (ii) outer curved surface area,
   (iii) total surface area.

Fig. 13.10

Fig. 13.11