# NCERT Class 9 Solutions: Surface Areas and Volumes (Chapter 13) Exercise 13.4 – Part 3 (For CBSE, ICSE, IAS, NET, NRA 2023)

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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
- ()

- Outer radius (R)

Q-9 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:

i. The surface area of the sphere with radius

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

The radius of the cylinder and its height .

Curved surface of cylinder

iii. Ratio of the areas

- Ratio of area