The diameter of the moon is approximately one fourth of the diameter of the earth. Find the ratio of their surface areas.
Surface area of a shephe = 4πr2
Ratio of surface areas of spheres = square of ratio of their radius
Given, diameter of the moon is approximately one fourth of the diameter of the earth
∴ rm = 1/4 × re
⇒ rm : re = 1 : 4
Ratio of their surface area = 1 : 16
The front compound wall of a house is decorated by wooden spheres of diameter 21 cm, placed on small supports as shown in Fig. 21.11. Eight such spheres are used for this purpose, and are to be painted silver. Each support is a cylinder of a 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 cm2 and black paint cost 5 paise per cm2.