RD Sharma Solutions for Class 9 Maths Chapter 20 Surface Area and Volume of a Right Circular Cone Exercise 20.1

RD Sharma Solutions for Class 9 Chapter 20 Surface Area and Volume of a Right Circular Cone Exercise 20.1 are provided here. These solutions have been prepared by our subject experts, and they provide accurate answers to all the exercise questions to which students can refer and be ready to solve questions at the time of their examinations. This chapter of RD Sharma Class 9  introduces students to a lot of questions related to the surface area of a right circular cone.
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RD Sharma Solutions for Class 9 Maths Chapter 20 Surface Area and Volume of a Right Circular Cone Exercise 20.1

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Access Answers to RD Sharma Solutions for Class 9 Maths Chapter 20 Surface Area and Volume of a Right Circular Cone Exercise 20.1 Page Number 20.7

Question 1: Find the curved surface area of a cone, if its slant height is 60 cm and the radius of its base is 21 cm.

Solution:

The slant height of cone (l) = 60 cm

The radius of the base of the cone (r) = 21 cm

Now,

Curved surface area of the right circular cone = πrl = 22/7 x 21 x 60 = 3960 cm²

Therefore the curved surface area of the right circular cone is 3960 cm²

Question 2: The radius of a cone is 5cm and the vertical height is 12cm. Find the area of the curved surface.

Solution:

The radius of cone (r) = 5 cm

Height of cone (h) = 12 cm

Find the slant height of cone (l):

We know, l² = r² + h²

l² = 5² +12²

l² = 25 + 144 = 169

Or l = 13 cm

Now,

C.S.A = πrl =3.14 x 5 x 13 = 204.28

Therefore, the curved surface area of the cone is 204.28 cm²

Question 3: The radius of a cone is 7 cm and the area of the curved surface is 176 cm².Find the slant height.

Solution:

Radius of cone(r) = 7 cm

Curved surface area(C.S.A)= 176cm²

We know, C.S.A. = πrl

⇒πrl = 176

⇒ 22/7 x 7 x l = 176

or l = 8

Therefore, the slant height of the cone is 8 cm.

Question 4: The height of a cone is 21 cm. Find the area of the base if the slant height is 28 cm.

Solution:

Height of cone(h) = 21 cm

The slant height of cone (l) = 28 cm

We know that, l² = r² + h²

28²=r²+21²

r²=28²−21²

or r= 7√7 cm

Now,

Area of the circular base = πr²

= 22/7 x (7√7 )²

=1078

Therefore, the area of the base is 1078 cm².

Question 5: Find the total surface area of a right circular cone with radius 6 cm and height 8 cm.

Solution:

The radius of cone (r) = 6 cm

Height of cone (h) = 8 cm

The total Surface area of the cone (T.S.A)=?

Find the slant height of the cone:

We know, l² = r² + h²

=6²+8²

= 36 + 64

= 100

or l = 10 cm

Now,

Total Surface area of the cone (T.S.A) = Curved surface area of cone + Area of circular base

= πrl + πr²

= (22/7 x 6 x 10) + (22/7 x 6 x 6)

= 1320/7 + 792/7

= 301.71

Therefore, the area of the base is 301.71 cm².

Question 6: Find the curved surface area of a cone with base radius 5.25 cm and slant height 10 cm.

Solution:

Base radius of the cone(r) = 5.25 cm

Slant height of the cone(l) = 10 cm

Curved surface area (C.S.A) = πrl

=22/7 x 5.25 x 10

= 165

Therefore, the curved surface area of the cone is 165 cm2.

Question 7: Find the total surface area of a cone, if its slant height is 21 m and the diameter of its base is 24 m.

Solution:

Diameter of the cone(d)=24 m

So, radius of the cone(r)= diameter/ 2 = 24/2 m = 12m

Slant height of the cone(l) = 21 m

T.S.A = Curved surface area of cone + Area of the circular base

= πrl+ πr²

= (22/7 x 12 x 21) + (22/7 x 12 x 12)

= 1244.57

Therefore, the total surface area of the cone is 1244.57 m².

Question 8: The area of the curved surface of a cone is 60 π cm². If the slant height of the cone is 8 cm, find the radius of the base.

Solution:

Curved surface area(C.S.A)= 60 π cm²

Slant height of the cone(l) = 8 cm

We know, Curved surface area(C.S.A )=πrl

⇒ πrl = 60 π

⇒ r x 8 = 60

or r = 60/8 = 7.5

Therefore, the radius of the base of the cone is 7.5 cm.

Question 9: The curved surface area of a cone is 4070 cm² and the diameter is 70 cm. What is its slant height? (Use π =22/7)

Solution:

Diameter of the cone(d) = 70 cm

So, radius of the cone(r)= diameter/2 = 70/2 cm = 35 cm

Curved surface area = 4070 cm²

Now,

We know, Curved surface area = πrl

So, πrl = 4070

By substituting the values, we get

22/7 x 35 x l = 4070

or l = 37

Therefore, the slant height of the cone is 37 cm.

Question 10: The radius and slant height of a cone are in the ratio 4:7. If its curved surface area is 792 cm², find its radius. (Use π =22/7)

Solution:

Curved surface area = 792 cm²

The radius and slant height of a cone are in the ratio 4:7 (Given)

Let 4x be the radius and 7x be the height of the cone.

Now,

Curved surface area (C.S.A.) = πrl

So, 22/7 x (4x) x (7x) = 792

or x² = 9

or x = 3

Therefore, radius = 4x = 4(3) cm = 12 cm

RD Sharma Solutions for Class 9 Maths Chapter 20 Surface Area and Volume of a Right Circular Cone Exercise 20.1

RD Sharma Solutions Class 9 Maths Chapter 20 Surface Area and Volume of a Right Circular Cone Exercise 20.1 is based on the following topics:

• Right Circular Cone Introduction
• Surface Area of a Right Circular Cone