Introduction

Chapter -13

Surface areas and volumes

Introduction
From Class IX, you are familiar with some of the solids like cuboid, cone, cylinder, and sphere (see Fig.). You have also learnt how to find their surface areas and volumes.

In our day-to-day life, we come across a number of solids made up of combinations of two or more of the basic solids as shown above.

Surface Area of a Combination of Solids
Let us consider the container seen in Fig. How do we find the surface area of such a solid? Now, whenever we come across a new problem, we first try to see, if we can break it down into smaller problems, we have earlier solved. We can see that this solid is made up of a cylinder with two hemispheres stuck at either end. It would look like what we have in Fig., after we put the pieces all together.

If we consider the surface of the newly formed object, we would be able to see only the curved surfaces of the two hemispheres and the curved surface of the cylinder. So, the total surface area of the new solid is the sum of the curved surface areas of each of the individual parts. This gives,
TSA of new solid = CSA of one hemisphere + CSA of cylinder + CSA of other hemisphere
Where TSA, CSA stand for ‘Total Surface Area’ and ‘Curved Surface Area’ respectively.
Let us now consider another situation. Suppose we are making a toy by putting together a hemisphere and a cone. Let us see the steps that we would be going through.
First, we would take a cone and a hemisphere and bring their flat faces together. Here, of course, we would take the base radius of the cone equal to the radius of the hemisphere, for the toy is to have a smooth surface. So, the steps would be as shown in Fig.

At the end of our trial, we have got ourselves a nice round-bottomed toy. Now if we want to find how much paint we would require to colour the surface of this toy, what would we need to know? We would need to know the surface area of the toy, which consists of the CSA of the hemisphere and the CSA of the cone.
So, we can say:
Total surface area of the toy = CSA of hemisphere + CSA of cone
Now, let us consider some examples.