Bihar Board Class 9th Maths Solutions Chapter 13 Surface Areas and Volumes Ex 13.4 Textbook Questions and Answers.

## BSEB Bihar Board Class 9th Maths Solutions Chapter 13 Surface Areas and Volumes Ex 13.4

Question 1.

Find the surface area of a sphere of radius : (i) 10.5 cm (ii) 5.6 cm (iii) 14 cm

Solution:

(i) We have : r – radius of the sphere = 10.5 cm

Surface area = 4πr²

= ( 4 x \(\frac { 22 }{ 7 }\) x 10.5 x 10.5 )cm² = 1386 cm²

(ii) We have : r = radius of the sphere = 5.6 cm

Surface area = 4πr²

= (4 x \(\frac { 22 }{ 7 }\) x 5.6 x 5.6) x cm²

= 394.24 cm²

(iii) We have : r = radius of the sphere = 14 cm

Surface area = (4 x \(\frac { 22 }{ 7 }\) x 14 x 14) cm² = 2464 cm²

Question 2.

Find the surface area of a sphere of diameter :

(i) 14 cm

(ii) 21 cm

(iii) 3.5 m

Solution:

(i) Here r = \(\frac { 14 }{ 2 }\) cm = 7 cm

Surface area = 4πr²

= (4 x \(\frac { 22 }{ 7 }\) x 7 x 7) cm² = 616 cm²

(ii) Here r = \(\frac { 21 }{ 2 }\) cm = 10.5 cm

∴ Surface area = 4πr²

= (4 x \(\frac { 22 }{ 7 }\) x 10.5 x 10.5) cm² = 1386 cm²

(iii) Here r = \(\frac { 3.5 }{ 2 }\) cm = 1.75 cm

∴ Surface area = 4πr²

= (4 x \(\frac { 22 }{ 7 }\) x 1.75 x 1.75) cm² = 38.5 cm²

Question 3.

Find the total surface area of a hemisphere of radius 10 cm. (Use π = 3.14)

Solution:

Here r = 10 cm

Total surface area of hemisphere

= 3πr² = (3 x 3.14 x 10 x 10) cm²

= 942 cm²

Question 4.

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:

Let r_{1} and r_{2} be the radius of balloons in the two cases.

Here r_{1} = 7 cm and r_{2} = 14 cm

Thus, the required ratio of their surface areas =1 : 4

Question 5.

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 Rs 16 per 100 cm². ‘

Solution:

Here r = (\(\frac { 10.5 }{ 2 }\)) cm = 5.25 cm

Curved surface area of the hemisphere

= 2πr² = (2 x \(\frac { 22 }{ 7 }\) x 5.25 x 5.25) cm²

= 173.25 cm²

Rate of tin-plating is Rs 16 per 100 cm²

∴ Cost of tin-plating the hemisphere

= Rs ( 173.25 x \(\frac { 16 }{ 100 }\)] = Rs 27.72

Question 6.

Find the radius of a sphere whose surface area is 154 cm².

Solution:

Let r be the radius of the sphere.

Thus, the radius of the sphere is 3.5 cm.

Question 7.

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 be R and that of the moon will be \(\frac { R }{ 4 }\)

The Radii of moon and earth are \(\frac { R }{ 8 }\) and \(\frac { R }{ 2 }\) respectively.

Question 8.

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, r = 5 cm

Thickness of steel = 0.25 cm

∴ Outer radius, R = (r + 0.25) cm = (5 + 0.25) cm = 5.25

∴ Outer Curved surface= 2πR²

= (2 x \(\frac { 22 }{ 7 }\) x 5.25 x 5.25) cm²

= 173.25 cm².

Question 9.

A right circular cylinder just encloses a sphere of radius r (see figure).

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:

(i) The radius of the sphere r, so its surface area = 4πr²

(ii) Since the right circular cylinder just encloses a sphere of radius r. So, the radius of cylinder = r and its height = 2r

Curved surface of cylinder = 2πr (2r) [∵ r = r, h = r]

= 4πr²

(iii) Ratio of areas = 4πr² : 4πr² =1 : 1