NCERT Class 9 Solutions: Surface Areas and Volumes (Chapter 13) Exercise 13.8 – Part 4
Q9 Twenty seven solid iron spheres, each of radius and surface area are melted to form a sphere with surface area . Find the

Radius of the new sphere,

Ratio of
Solution:
When the spheres are melted the volume of molten material remains the same.

Volume of solid sphere of radius
Volume of the new sphere of radius …………….equation (2)
Since the volume must remain the same, therefore,


Required ratio of surface area,

=



or the ratio is
Q10 A capsule of medicine is in the shape of a sphere of diameter . How much medicine (in ) is needed to fill this capsule?
Solution:
Diameter of the spherical capsule
Therefore, radius (r)
Medicine needed for filling this capsule = Volume of this spherical capsule