Exercise 13.4
Assume π = 22/7 , unless stated otherwise.
Q1. Find the surface area of a sphere of radius:
(i) 10.5 cm (ii) 5.6 cm (iii) 14 cm
Solution:
Q2. Find the surface area of a sphere of diameter:
(i) 14 cm (ii) 21 cm (iii) 3.5 m
Solution:
(i) Surface area of sphere = 4πr2
Q3. Find the total surface area of a hemisphere of radius 10 cm. (Use π = 3.14)
Solution:
Total surface area of hemisphere = 3πr2
⇒ 3 × 3.14 × 10 × 10
⇒ 3 × 314
⇒ 942 cm2
Q4. The radius of a spherical balloon increases from 7 cm to 14 cm as air is being pumped into it. Find the ratio of surface areas of the balloon in the two cases.
Solution:
Surface area of sphere = 4πr2
Q5. A hemispherical bowl made of brass has inner diameter 10.5 cm. Find the cost of tin-plating it on the inside at the rate of 16 per 100 cm2.
Solution:
Curved surface area of hemisphere = 2πr2
Q6. Find the radius of a sphere whose surface area is 154 cm2.
Solution:
Area = 154 cm2
Surface area of sphere = 4πr2
Q7. The diameter of the moon is approximately one fourth of the diameter of the earth. Find the ratio of their surface areas.
Solution:
Let the diameter of earth = x
Q8. A hemispherical bowl is made of steel, 0.25 cm thick. The inner radius of the bowl is 5 cm. Find the outer curved surface area of the bowl.
Solution:
Inner radius = 5 cm, width = 0.25 cm, radius = 5.25 cm
Curved surface area of hemisphere = 2πr2
⇒ 44 × 0.75 × 5.25
⇒ 173.25
Q9. A right circular cylinder just encloses a sphere of
radius r (see Fig. 13.22). Find
(i) surface area of the sphere,
(ii) curved surface area of the cylinder,
(iii) ratio of the areas obtained in (i) and (ii).
Solution:
Radius of sphere = r, radius of cylinder = r + r = 2r
(i) surface area of sphere = 4πr2
(ii) curved surface area of cylinder = 2πrh
⇒ 2πr(2r)
⇒ 4πr2