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Class 9 Maths Question Answers - Surface Area and Volume of a Right Circular Cone

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Q u e s t i o n : 3 9
The height of a cone is 15 cm. If its volume is 500 p
cm
3
, then find the radius of its base.
Page 2


                          
                 
         
                           
                     
                    
    
                            
  
  
 
 
          
              
     
 
 
 
 
         
                  
    
                     
  
 
                     
Q u e s t i o n : 3 9
The height of a cone is 15 cm. If its volume is 500 p
cm
3
, then find the radius of its base.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone = 
It is given that the height of the cone is ‘h’ = 15 cm and that the volume of the cone is 
We can now find the radius of base ‘r’ by using the formula for the volume of a cone.
= 
= 
= 100
= 10
Hence the radius of the base of the given cone is 
Q u e s t i o n : 4 0
If the volume of a right circular cone of height 9 cm is 48 p
cm
3
, find the diameter of its base.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone = 
It is given that the height of the cone is ‘h’ = 9 cm and that the volume of the cone is 48 p cm
3
We can now find the radius of base ‘r’ by using the formula for the volume of a cone.
= 
= 
= 16
= 4
Hence the radius of the base of the cone with given dimensions is ‘r’ = 4 cm.
The diameter of base is twice the radius of the base.
Hence the diameter of the base of the cone is 
Q u e s t i o n : 4 1
If the height and slant height of a cone are 21 cm and 28 cm respectively. Find its volume.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone= 
The vertical height is given as ‘h’ = 21 cm, and the slant height is given as ‘l’ = 28 cm.
To find the base radius ‘r’ we use the relation between r, l and h.
We know that in a cone 
Page 3


                          
                 
         
                           
                     
                    
    
                            
  
  
 
 
          
              
     
 
 
 
 
         
                  
    
                     
  
 
                     
Q u e s t i o n : 3 9
The height of a cone is 15 cm. If its volume is 500 p
cm
3
, then find the radius of its base.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone = 
It is given that the height of the cone is ‘h’ = 15 cm and that the volume of the cone is 
We can now find the radius of base ‘r’ by using the formula for the volume of a cone.
= 
= 
= 100
= 10
Hence the radius of the base of the given cone is 
Q u e s t i o n : 4 0
If the volume of a right circular cone of height 9 cm is 48 p
cm
3
, find the diameter of its base.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone = 
It is given that the height of the cone is ‘h’ = 9 cm and that the volume of the cone is 48 p cm
3
We can now find the radius of base ‘r’ by using the formula for the volume of a cone.
= 
= 
= 16
= 4
Hence the radius of the base of the cone with given dimensions is ‘r’ = 4 cm.
The diameter of base is twice the radius of the base.
Hence the diameter of the base of the cone is 
Q u e s t i o n : 4 1
If the height and slant height of a cone are 21 cm and 28 cm respectively. Find its volume.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone= 
The vertical height is given as ‘h’ = 21 cm, and the slant height is given as ‘l’ = 28 cm.
To find the base radius ‘r’ we use the relation between r, l and h.
We know that in a cone 
= 
= 
= 
Therefore the base radius is, r =  cm.
Substituting the values of r =  cm and h = 21 cm in the formula for volume of a cone.
Volume = 
p r
2
h
3
=
p ×
v
343
2
×21
3
=2401
p
Hence the volume of the given cone with the specified dimensions is
2401 p cm
3
.
Q u e s t i o n : 4 2
The height  of a conical vessel is 3.5 cm. If its capacity is 3.3 litres of milk. Find its diameter of its base.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone = 
It is given that the height of the cone is ‘h’ = 3.5 cm and that the volume of the cone is 3.3 liters
We know that,
1 liter = 1000 cubic centimeter
Hence, the volume of the cone in cubic centimeter is . 
We can now find the radius of base ‘r’ by using the formula for the volume of a cone, while using 
= 
= 
= 900
= 30
Hence the radius of the base of the cone with given dimensions is ‘r’ = 30 cm.
The diameter of base is twice the radius of the base.
Hence the diameter of the base of the cone is 
Q u e s t i o n : 4 3
If the radius and slant height of a cone are in the ratio 7 : 13 and its curved surface area is 286 cm
2
, find its radius.
S o l u t i o n :
It is given that the curved surface area
( )
Page 4


                          
                 
         
                           
                     
                    
    
                            
  
  
 
 
          
              
     
 
 
 
 
         
                  
    
                     
  
 
                     
Q u e s t i o n : 3 9
The height of a cone is 15 cm. If its volume is 500 p
cm
3
, then find the radius of its base.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone = 
It is given that the height of the cone is ‘h’ = 15 cm and that the volume of the cone is 
We can now find the radius of base ‘r’ by using the formula for the volume of a cone.
= 
= 
= 100
= 10
Hence the radius of the base of the given cone is 
Q u e s t i o n : 4 0
If the volume of a right circular cone of height 9 cm is 48 p
cm
3
, find the diameter of its base.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone = 
It is given that the height of the cone is ‘h’ = 9 cm and that the volume of the cone is 48 p cm
3
We can now find the radius of base ‘r’ by using the formula for the volume of a cone.
= 
= 
= 16
= 4
Hence the radius of the base of the cone with given dimensions is ‘r’ = 4 cm.
The diameter of base is twice the radius of the base.
Hence the diameter of the base of the cone is 
Q u e s t i o n : 4 1
If the height and slant height of a cone are 21 cm and 28 cm respectively. Find its volume.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone= 
The vertical height is given as ‘h’ = 21 cm, and the slant height is given as ‘l’ = 28 cm.
To find the base radius ‘r’ we use the relation between r, l and h.
We know that in a cone 
= 
= 
= 
Therefore the base radius is, r =  cm.
Substituting the values of r =  cm and h = 21 cm in the formula for volume of a cone.
Volume = 
p r
2
h
3
=
p ×
v
343
2
×21
3
=2401
p
Hence the volume of the given cone with the specified dimensions is
2401 p cm
3
.
Q u e s t i o n : 4 2
The height  of a conical vessel is 3.5 cm. If its capacity is 3.3 litres of milk. Find its diameter of its base.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone = 
It is given that the height of the cone is ‘h’ = 3.5 cm and that the volume of the cone is 3.3 liters
We know that,
1 liter = 1000 cubic centimeter
Hence, the volume of the cone in cubic centimeter is . 
We can now find the radius of base ‘r’ by using the formula for the volume of a cone, while using 
= 
= 
= 900
= 30
Hence the radius of the base of the cone with given dimensions is ‘r’ = 30 cm.
The diameter of base is twice the radius of the base.
Hence the diameter of the base of the cone is 
Q u e s t i o n : 4 3
If the radius and slant height of a cone are in the ratio 7 : 13 and its curved surface area is 286 cm
2
, find its radius.
S o l u t i o n :
It is given that the curved surface area
( )
C. S. A of the cone is 286 cm
2 
and that the ratio between the base radius and the slant height is 7: 13. The formula of the
curved surface area of a cone with base radius ‘r’ and slant height ‘l’ is given as
Curved Surface Area = prl
Since only the ratio between the base radius and the slant height is given, we shall use them by introducing a constant ‘k’
So, r = 7k
l = 13k
Substituting the values of C.S.A, base radius, slant height and using  in the above equation,
Curved Surface Area, 286 = 
286 = 286 k
2
1 = k
2
Hence the value of k = 1
From this we can find the value of base radius,
r = 7k
r = 7
Therefore the base radius of the cone is 
Q u e s t i o n : 4 4
Find the area of canvas required for a conical tent of height 24 m and base radius 7 m.
S o l u t i o n :
The amount of canvas required to make a cone would be equal to the curved surface area of the cone.
The formula of the curved surface area of a cone with base radius ‘r’ and slant height ‘l’ is given as 
Curved Surface Area = 
It is given that the vertical height ‘h’ = 24 m and base radius ‘r’ = 7 m.
To find the slant height ‘l’ we use the following relation
Slant height, l = 
= 
= 
= 
l = 25
Hence the slant height of the given cone is 25 m.
Now, substituting the values of r = 7 m and slant height l = 25 m and using  in the formula of C.S.A,
We get 
Curved Surface Area = 
=
22
25
= 550 
Page 5


                          
                 
         
                           
                     
                    
    
                            
  
  
 
 
          
              
     
 
 
 
 
         
                  
    
                     
  
 
                     
Q u e s t i o n : 3 9
The height of a cone is 15 cm. If its volume is 500 p
cm
3
, then find the radius of its base.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone = 
It is given that the height of the cone is ‘h’ = 15 cm and that the volume of the cone is 
We can now find the radius of base ‘r’ by using the formula for the volume of a cone.
= 
= 
= 100
= 10
Hence the radius of the base of the given cone is 
Q u e s t i o n : 4 0
If the volume of a right circular cone of height 9 cm is 48 p
cm
3
, find the diameter of its base.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone = 
It is given that the height of the cone is ‘h’ = 9 cm and that the volume of the cone is 48 p cm
3
We can now find the radius of base ‘r’ by using the formula for the volume of a cone.
= 
= 
= 16
= 4
Hence the radius of the base of the cone with given dimensions is ‘r’ = 4 cm.
The diameter of base is twice the radius of the base.
Hence the diameter of the base of the cone is 
Q u e s t i o n : 4 1
If the height and slant height of a cone are 21 cm and 28 cm respectively. Find its volume.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone= 
The vertical height is given as ‘h’ = 21 cm, and the slant height is given as ‘l’ = 28 cm.
To find the base radius ‘r’ we use the relation between r, l and h.
We know that in a cone 
= 
= 
= 
Therefore the base radius is, r =  cm.
Substituting the values of r =  cm and h = 21 cm in the formula for volume of a cone.
Volume = 
p r
2
h
3
=
p ×
v
343
2
×21
3
=2401
p
Hence the volume of the given cone with the specified dimensions is
2401 p cm
3
.
Q u e s t i o n : 4 2
The height  of a conical vessel is 3.5 cm. If its capacity is 3.3 litres of milk. Find its diameter of its base.
S o l u t i o n :
The formula of the volume of a cone with base radius ‘r’ and vertical height ‘h’ is given as
Volume of cone = 
It is given that the height of the cone is ‘h’ = 3.5 cm and that the volume of the cone is 3.3 liters
We know that,
1 liter = 1000 cubic centimeter
Hence, the volume of the cone in cubic centimeter is . 
We can now find the radius of base ‘r’ by using the formula for the volume of a cone, while using 
= 
= 
= 900
= 30
Hence the radius of the base of the cone with given dimensions is ‘r’ = 30 cm.
The diameter of base is twice the radius of the base.
Hence the diameter of the base of the cone is 
Q u e s t i o n : 4 3
If the radius and slant height of a cone are in the ratio 7 : 13 and its curved surface area is 286 cm
2
, find its radius.
S o l u t i o n :
It is given that the curved surface area
( )
C. S. A of the cone is 286 cm
2 
and that the ratio between the base radius and the slant height is 7: 13. The formula of the
curved surface area of a cone with base radius ‘r’ and slant height ‘l’ is given as
Curved Surface Area = prl
Since only the ratio between the base radius and the slant height is given, we shall use them by introducing a constant ‘k’
So, r = 7k
l = 13k
Substituting the values of C.S.A, base radius, slant height and using  in the above equation,
Curved Surface Area, 286 = 
286 = 286 k
2
1 = k
2
Hence the value of k = 1
From this we can find the value of base radius,
r = 7k
r = 7
Therefore the base radius of the cone is 
Q u e s t i o n : 4 4
Find the area of canvas required for a conical tent of height 24 m and base radius 7 m.
S o l u t i o n :
The amount of canvas required to make a cone would be equal to the curved surface area of the cone.
The formula of the curved surface area of a cone with base radius ‘r’ and slant height ‘l’ is given as 
Curved Surface Area = 
It is given that the vertical height ‘h’ = 24 m and base radius ‘r’ = 7 m.
To find the slant height ‘l’ we use the following relation
Slant height, l = 
= 
= 
= 
l = 25
Hence the slant height of the given cone is 25 m.
Now, substituting the values of r = 7 m and slant height l = 25 m and using  in the formula of C.S.A,
We get 
Curved Surface Area = 
=
22
25
= 550 
Therefore the Curved Surface Area of the cone is 
Q u e s t i o n : 4 5
Find the area of metal sheet required in making a closed hollow cone of base radius 7 cm and height 24 cm.
S o l u t i o n :
The area of metal sheet required to make this hollow closed cone would be equal to the total surface area of the cone.
The formula of the total surface area of a cone with base radius ‘r’ and slant height ‘l’ is given as 
Total Surface Area = 
It is given that the vertical height ‘h’ = 24 cm and base radius ‘r’ = 7 cm.
To find the slant height ‘l’ we use the following relation
Slant height, l = 
= 
= 
= 
l = 25
Hence the slant height of the given cone is 25 cm.
Now, substituting the values of r = 7 cm and slant height l = 25 cm and using  in the specified formula,
Total Surface Area = 
=
22
32
= 704
Therefore the total area of the metal sheet required to make the closed hollow cone is equal to 
Q u e s t i o n : 4 6
Find the length of cloth used in making a conical pandal of height 100 m and base radius 240 m, if the cloth is 100 p
m wide.
S o l u t i o n :
The area of cloth required to make the conical pandal would be equal to the curved surface area of the cone.
The formula of the curved surface area of a cone with base radius ‘r’ and slant height ‘l’ is given as 
Curved Surface Area = prl
It is given that the vertical height ‘h’ = 100 m and base radius ‘r’ = 240 m.
To find the slant height ‘l’ we use the following relation
Slant height, l = 
= 
=  
= 
l = 260
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