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A hemi-spherical dome of a building needs to be painted. IF the circumference of the base of the dome is 17.6 m, find the cost of painting it, given the cost of painting is Rs. 5 per 100 cm2.
Circumference of a circle = 2πr
Surface area of a hemisphere = 2πr2
Given, base of the dome is 17.16 m
⇒ 2 × (22/7) × r = 17.6
⇒ r = 2.8 m
Surface area of the hemisphere = 2 × (22/7) × 2.82 = 49.28 m2 = 492800 cm2
Cost of painting is Rs. 5 per 100 cm2,
Cost of painting the dome = (492800/100) × 5 = Rs. 24640
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.