First, he should take a cone and hemisphere of equal radii and bring their flat faces together.Here, of course, he should take the base radius of the cone equal to the radius of the hemisphere, for the toy to have a smooth surface. So, the steps would be as shown below:

At the end, he got a nice round-bottomed toy. Now, if he wants to find how much paint he is required to colour the surface of the toy, for this he needs to know the total surface area of the toy, which consists of the CSA of the hemisphere and the CSA of the cone.

TSA of the toy = CSA of hemisphere + CSA of cone

Try This
- Use known solid shapes and make as many objects (by combining more than two) as possible that you come across in your daily life.
[Hint : Use clay, or balls, pipes, paper cones, boxes like cube, cuboid etc]

THINK AND DISCUSS
A sphere is inscribed in a cylinder. Is the surface of the sphere equal to the curved surface of the cylinder? If yes, explain how?


Example-8. A right triangle, whose base and height are 15 cm. and 20 cm. respectively is made to revolve about its hypotenuse. Find the volume and surface area of the double cone so formed.
Solution :Let ABC be the right angled triangle such that

AB = 15cm and AC = 20 cm

Using Pythagoras theorem in DABC we have BC2= AB2 + AC2 pg no:256


page no:256

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