A wooden toy is in the form of a cone surmounted on a hemisphere. The diameter of the base of the cone is 16 cm and its height is 15 cm. Find the cost of painting the toy at Rs. 7 per 100 cm2.
From the figure,
Radius of the hemisphere = 8 cm
Height of the cone = 15 cm
Lateral length of the cone = √(h2 + r2) = √(152 + 82) = 17 cm
Curved surface area of hemisphere = 2πr2
Curved surface area of the cone = πrl
Total curved surface area of the toy = 2πr2 + πrl
⇒ Total curved surface area of the toy = 2 × (22/7) × 82 + (22/7) × 8 × 17
⇒ Total curved surface area of the toy = 829.714 cm2
Cost of painting the toy at Rs. 7 per 100 cm2
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.