RBSE Solutions for Class 9 Maths Chapter 13 Surface Areas and Volumes Ex 13.4

Rajasthan Board RBSE Solutions for Class 9 Maths Chapter 13 Surface Areas and Volumes Ex 13.4 Textbook Exercise Questions and Answers.

Rajasthan Board RBSE Solutions for Class 9 Maths in Hindi Medium & English Medium are part of RBSE Solutions for Class 9. Students can also read RBSE Class 9 Maths Important Questions for exam preparation. Students can also go through RBSE Class 9 Maths Notes to understand and remember the concepts easily. Practicing the class 9 math chapter 13 hindi medium textbook questions will help students analyse their level of preparation.

RBSE Class 9 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
Answer;
(i) We have : r = radius of the sphere = 10.5 cm
∴ Surface area = 4πr2 = (4 × \(\frac{22}{7}\) × 10.5 × 10.5) cm2 = 1386 cm2
(ii) We have : r = radius of the sphere = 5.6 cm
∴ Surface area = 4πr2 = (4 × \(\frac{22}{7}\) × 5.6 × 5.6) cm2 = 394.24 cm2
(iii) We have : r = radius of the sphere = 14 cm
∴ Surface area = 14 × \(\frac{22}{7}\) × 14 × 14) = 2464 cm2.

RBSE Solutions for Class 9 Maths Chapter 13 Surface Areas and Volumes Ex 13.4

Question 2.
Find the surface area of a sphere of diameter : .
(i) 14 cm
(ii) 21 cm
(iii) 3.5 m
Answer:
(i) Here r = \(\left(\frac{14}{2}\right)\)cm = 7 cm2.
Surface area = 4πr= (4 × \(\frac{22}{7}\) × 7 × 7) cm2 = 616 cm2

(ii) Here r = \(\left(\frac{21}{2}\right)\) cm = 10.5 cm
Surface area = 4πr2 = (4 × \(\frac{22}{7}\) × 10.5 × 10.5)cm2 = 1386 cm2

(iii) Here r = cm = 1.75 cm
Surface area = 4πr2 = (4 × \(\frac{22}{7}\) × 1.75 × 1.75)cm2 = 38.5 cm2.

Question 3.
Find the total surface area of a hemisphere of radius 10 cm. (Use π = 3.14)
Answer:
Here r = 10 cm
Total surface area of hemisphere = 3πr2
= (3 × 3.14 × 10 × 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.
Answer:
Let r1 and r2 be the radii of balloons in the two cases.
Here r1 = 7 cm and r2 = 14 cm
∴ Ratio of their surface areas = \(\frac{4 \pi r_{1}^{2}}{4 \pi r_{2}^{2}}=\frac{r_{1}^{2}}{r_{2}^{2}}=\frac{7 \times 7}{14 \times 14}=\frac{1}{4}\)
Thus, the required ratio of their surface areas is 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 ₹ 16 per 100 cm2.
Answer:
Here r = \(\left(\frac{10.5}{2}\right)\) cm = 5.25 cm
Curved surface area of the hemisphere = 2πr2 = (2 × \(\frac{22}{7}\) × 5.25 × 5.25)
= 173.25 cm2
Rate of tin-plating is ₹ 16 per 100 cm2.
∴ Cost of tin-plating the inside of the hemisphere
Thus, the radius of the sphere is 3.5 cm.

RBSE Solutions for Class 9 Maths Chapter 13 Surface Areas and Volumes Ex 13.4

Question 6.
Fiüd the radius of a sphere whose surface area is 154 cm2.
Answer:
Let r be the radius of the sphere.
Surface area = 154 cm2 ⇒ 4πr2 = 154
So, 4 × \(\frac{22}{7}\) × r2 = 154 ⇒ r2 = \(\frac{154 \times 7}{4 \times 22}=\frac{49}{4}\)
r = \(\frac{7}{2}\) =3.5
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.
Answer:
Let the diameter of earth be R and that of the moon will be \(\frac{R}{2}\)
∴ Ratio of their Surface areas = \(\frac{4 \pi\left(\frac{R}{8}\right)^{2}}{4 \pi\left(\frac{R}{2}\right)^{2}}=\frac{\frac{1}{64}}{\frac{1}{4}}\)
= \(\frac{1}{64} \times \frac{4}{1}=\frac{1}{16}\), i.e 1 : 16

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.
Answer:
Inner radius r = 5 cm
Thickness of steel = 0.25 cm
∴ Outer radius R = (r + 0.25) cm = (5 + 0.25) cm = 5.25 cm
∴ Outer curved surface area = 2πR2
= (2 ×\( \frac{22}{7}\) × 5.25 × 5.25)cm2 = 173.25 cm2

RBSE Solutions for Class 9 Maths Chapter 13 Surface Areas and Volumes Ex 13.4

Question 9.
A right circular cylinder just encloses a sphere of radius r (see figure). Find :
RBSE Solutions for Class 9 Maths Chapter 13 Surface Areas and Volumes Ex 13.4 1
(i) surface area of the sphere,
(ii) curved surface area of the cylinder,
(iii) ratio of the areas obtained in (i) and (ii).
Answer:
(i) The radius of the sphere is r. So, its surface area = 4πr2.
(ii) Since, the right circular cylinder just encloses a sphere of radius r, therefore the radius of cylinder = r and its height = 2r.
Curved surface area of cylinder = 2πr (2r) (∵ r = r, h = r)
= 4πr2
(iii) Ratio of areas= 4πr2 : 4πr2 =1:1.

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Last Updated on April 19, 2022, 3:06 p.m.
Published April 19, 2022