SURFACE AREAS AND VOLUMES 237
- The front compound wall of a house is
decorated by wooden spheres of diameter 21
cm, placed on small supports as shown in Fig
13.32. Eight such spheres are used for this
purpose, and are to be painted silver. Each
support is a cylinder of radius 1.5 cm and height
7 cm and is to be painted black. Find the cost
of paint required if silver paint costs 25 paise
per cm^2 and black paint costs 5 paise per cm^2. - The diameter of a sphere is decreased by 25%. By what per cent does its curved
surface area decrease?
13.10 Summary
In this chapter, you have studied the following points:
- Surface area of a cuboid = 2 (lb + bh + hl)
- Surface area of a cube = 6a^2
- Curved surface area of a cylinder = 2✁✁rh
- Total surface area of a cylinder = 2✁✁r(r + h)
- Curved surface area of a cone = ✁rl
- Total surface area of a right circular cone = ✁✁✁✁rl + ✁✁r^2 , i.e.,✁✁✁✁r (l + r)
- Surface area of a sphere of radius r = 4 ✁✁ r^2
- Curved surface area of a hemisphere = 2✁r^2
- Total surface area of a hemisphere = 3✁✁✁✁r^2
- Volume of a cuboid = l × b × h
- Volume of a cube = a^3
- Volume of a cylinder = ✁✁✁✁r^2 h
- Volume of a cone =
1
3
✁r^2 h- Volume of a sphere of radius r =^3
4
3
r- Volume of a hemisphere =^3
2
3
r[Here, letters l, b, h, a, r, etc. have been used in their usual meaning, depending on the
context.]
Fig. 13.32