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RD Chapter 21 Surface Areas and Volume of a Sphere Ex 21.1 Solutions

Question - 11 : - 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 ₹7 per 100 cm2.

Answer - 11 : -

Diameter of toy = 16 cm
Radius (r) = 16/2 = 8 cm
Height of conical part (h) = 15 cm

Question - 12 : - A storage tank consists of a circular cylinder with a hemisphere adjoined on either end. If the external diameter of the cylinder be 1.4 m and its length be 8 m, find the cost of painting it on the outside at the rate of ₹10 per m2.

Answer - 12 : -

Diameter of the tank = 1.4 m
∴ Radius (r) = 1.4/2 m = 0.7 m
and height of cylindrical portion = 8m

Question - 13 : - The front compound wall of a house is decorated by wooden spheres of diameter 21 cm, placed on small supports as shown in the figure. 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 cm2 and black paint costs 5 paise per cm2. [NCERT]

Answer - 13 : - Diameter of each spheres = 21 cm
∴ Radius (R) = 21/2 cm
Radius of each cylinder (r) = 1.5 cm
and height (h) = 7 cm

Now surface area of one sphere = 4πR2


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