NCERT Class 9 Solutions: Surface Areas and Volumes (Chapter 13) Exercise 13.4 – Part 3

Q-7 The diameter of the moon is approximately one fourth of the diameter of the earth. Find the ratio of their surface areas.

Solution:

• Consider the diameter of earth be and that of the moon will be

• Radius of the earth

• Radius of the moon

Ratio of their surface area

So, the ratio of their surface areas.

Q-8 A hemispherical bowl is made of steel, thick. The inner radius of the bowl is. Find the outer curved surface area of the bowl.

Solution:

• Inner radius of the bowl (r)

• Thickness of the steel

  Outer radius (R)

Outer curved surface

• ()

Q-9 A right circular cylinder just encloses a sphere of radius r (see Fig). Find

1. Surface area of the sphere,

2. Curved surface area of the cylinder,

3. Ratio of the areas obtained in (i) and (ii).

a Right Circular Cylinder

A right circular cylinder, sphere of radius r

Solution:

1. The surface area of the sphere with radius

2. The right circular cylinder just encloses a sphere of radius

   The radius of the cylinder and its height .

   Curved surface of cylinder

3. Ratio of the areas

Ratio of area