# NCERT Class 9 Solutions: Surface Areas and Volumes (Chapter 13) Exercise 13.8 – Part 3

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Q-7 Find the volume of a sphere whose surface area is.

Solution:

Consider radius of the sphere to be

We know that, surface area

Since we know radius r, volume is =

Q-8 A dome of a building is in the form of a hemisphere. From inside, it was white-washed at the cost of If the cost of white-washing is per square metre, find the

1. Inside surface area of the dome.

2. Volume of the air inside the dome.

Solution:

We know that,

• Inside of the building is white-washed at the cost of

• Rate of white washing is per square metre.

I. Inside surface area of the dome

II. Let, radius of the dome be , then surface area (which we know from I. above)

Volume of the air inside the dome = Volume of the dome. However, since we know the radius, we can now calculate the volume =