NCERT Class 9 Solutions: Surface Areas and Volumes (Chapter 13) Exercise 13.4 – Part 3
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Q7 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.
Q8 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


()

Q9 A right circular cylinder just encloses a sphere of radius r (see Fig). Find

Surface area of the sphere,

Curved surface area of the cylinder,

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

The surface area of the sphere with radius

The right circular cylinder just encloses a sphere of radius
The radius of the cylinder and its height .
Curved surface of cylinder


Ratio of the areas
Ratio of area