# NCERT Class 9 Solutions: Surface Areas and Volumes (Chapter 13) Exercise 13.4 – Part 2

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Curved surface area of hemisphere is half the area of sphere. The total surface area also includes the base of radius *r* at the bottom therefore TSA of Sphere =

Q-3 Find the total surface area of a hemisphere of radius. (Use)Solution:

Hemisphere of radius

Therefore, total surface area of hemisphere = = =

Q-4 The radius of a spherical balloon increases from to as air is being pumped into it. Find the ratio of surface areas of the balloon in the two cases.Solution:

Now, the initial radius is and the increased radius of balloons is. Where,

Therefore, ratio of the surface area

So, the surface areas are in ratio .

Q-5 A hemispherical bowl made of brass has inner diameter . Find the cost of tin-plating it on the inside at the rate of .

Solution:

A bowl is a hemisphere with no base. So we are interested only in the curved surface area.

Radius of the bowl (r) ()

Therefore, curved surface area of the hemispherical bowl

Tin-plating costs for 100 cm^{2} is

So, cost of tin plating

Finally, total cost of tin-plating the hemisphere bowl with area cm^{2} is

Q-6 Find the radius of a sphere whose surface area is.Solution:

Consider, be the radius of the sphere.

We know that surface area =

Therefore,