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

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**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:

Slant height of cone (l) = 60 cm

Radius of the base of the cone (r) = 21 cm

Now,

Curved surface area of the right circular cone = Ï€rl = 22/7 x 21 x 60 = 3960 cm^{2}

Therefore the curved surface area of the right circular cone is 3960 cm^{2}

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

**Solution:**

Radius of cone (r) = 5 cm

Height of cone (h) = 12 cm

Find Slant Height of cone (l):

We know, l^{2} = âˆš r^{2} + h^{2}

l^{2 }= 5^{2 }+12^{2}

l^{2 }= 25 + 144 = 169

Or l = 13 cm

Now,

C.S.A = Ï€rl =3.14 x 5 x 12 = 204.28

Therefore, the curved surface area of the cone is 204.28 cm^{2}

**Question 3 : The radius of a cone is 7 cm and area of curved surface is 176 cm ^{2} .Find the slant height.**

**Solution:**

Radius of cone(r) = 7 cm

Curved surface area(C.S.A)= 176cm^{2}

We know, C.S.A. = Ï€rl

â‡’Ï€rl = 176

â‡’ 22/7 x 7 x l = 176

or l = 8

Therefore, slant height of the cone is 8 cm.

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

**Solution:**

Height of cone(h) = 21 cm

Slant height of cone (l) = 28 cm

We know that, l^{2 }= r^{2} + h^{2}

28^{2}=r^{2}+21^{2}

r^{2}=28^{2}âˆ’21^{2}

or r= 7âˆš7 cm

Now,

Area of the circular base = Ï€r^{2}

= 22/7 x (7âˆš7 )^{2}

=1078

Therefore, area of the base is 1078 cm^{2}.

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

**Solution:**

Radius of cone (r) = 6 cm

Height of cone (h) = 8 cm

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

Find slant height of cone:

We know, l^{2 }= r^{2} + h^{2}

=6^{2}+8^{2}

= 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

= Ï€rl + Ï€r^{2}

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

= 1320 + 7927

= 301.171

Therefore, area of the base is 301.71cm^{2}.

**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) = Ï€rl

=22/7 x 5.25 x 10

= 165

Therefore, curved surface area of the cone is 165cm^{2}.

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

= Ï€rl+ Ï€r^{2}

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

= 1244.57

Therefore, total surface area of the cone is 1244.57 m^{2}.

**Question 8: The area of the curved surface of a cone is 60 Ï€ cm ^{2}. If the slant height of the cone be 8 cm, find the radius of the base.**

**Solution:**

Curved surface area(C.S.A)= 60 Ï€ cm^{2}

Slant height of the cone(l) = 8 cm

Ee know, Curved surface area(C.S.A )=Ï€rl

â‡’ Ï€rl = 60 Ï€

â‡’ r x 8 = 60

or r = 60/8 = 7.5

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

**Question 9: The curved surface area of a cone is 4070 cm ^{2} and diameter is 70 cm .What is its slant height? (Use Ï€ =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 cm^{2}

Now,

We know, Curved surface area = Ï€rl

So, Ï€rl = 4070

By substituting the values, we get

22/7 x 35 x l = 4070

or l = 37

Therefore, slant height of 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 cm ^{2}, find its radius. (Use Ï€ =22/7)**

**Solution:**

Curved surface area = 792 cm^{2}

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 cone.

Now,

Curved surface area (C.S.A.) = Ï€rl

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

or x^{2 }= 9

or x = 3

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

## RD Sharma Solutions for Class 9 Maths Chapter 20 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