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

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Surface area of sphere=4πr^2

Surface Area of Sphere

Surface area of sphere=4πr^2

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