[Maths Class Notes] on Right Circular Cone Pdf for Exam

Slant Height of the Right Circular Cone

Consider a right circular cone of height AO = h, slant height AC = l, and radius OC = r. Angle AOC = 900.

By Pythagoras theorem, we have

AC² = AO² + OC²

l² = h² + r²

l = √(r² + h²)

It follows from this relation of height, slant height, and radius we can find any of them.

Surface Area of Right Circular Cone

The curved surface area of a cone is also called as the lateral surface area.

The curved surface area of a cone  S with radius r, height h, and slant height h is

S= π r l 

S = ½ ( 2πr) l

S = ½ ( circumference of base ) x (slant height)

Now, Total Surface area of right circular cone =  Curved surface area + Area of the base

= S + πr2
= πrl + πr2

= πr( l + r)

Volume of Right Circular Cone

To find the volume of Right Circular Cone with radius r and height h 

The volume of Right Circular Cone = 1/3 ? r² h

= 1/3 ? r2 h

= 1/3 x ( area of the base) x height

Now let us see some problems using the right circular cone formula. Memorize the equation of the right circular cone and solve the problems on the right circular cone easily.

Solved Examples

Example 1:The radius of a cone is 3 cm and the vertical height is 4cm. Find the area of the curved surface of the right circular cone.

Solution: We have r = 3cm, h = 4 cm

Let h cm be the slant height of the cone.

Then Slant height  [ = ;{r^2} + {h^2}]

[ = {3^2} + {4^2}]

= 9 + 16

= 25

l = 5cm

Area of the curved surface =πrl

[ = {text{ }}frac{{22}}{7} times 3 times 5]

= 47.14 cm2

Example 2: The diameter of a right circular cone is 8 cm and its volume is [48pi c{m^3}] What is its height?

Solution: Let h cm be the height of the cone.

The diameter of the cone = 8 cm

The radius of the cone = 4 cm

Volume of the cone = [48pi c{m^3}]

Now, 

[frac{1}{3} times pi  times 4 times 4 times h = 48pi ]

[h = frac{{48pi  times 3}}{{16pi cm}}]= 9cm

Hence the height of the cone is 9 cm. 

Example 3: A solid cube of side 7 cm is melted to make a cone of height 5 cm, find the radius of the base of the cone.

Solution: We have, 

Volume of the cube = [{left( {Side} right)^3} = {7^3}c{m^3} = 343{text{ }}c{m^3}]

Let the radius of the base be r cm. Then, 

Volume of the cone [ = {text{ }}frac{1}{3} times frac{{22}}{7} times {r^2} times 5{text{ }}c{m^3}]

Since the solid cube is melted to make a cone    

Volume of the cube = Volume of the cone

343 = ⅓ x 22/7 x r2 x 5

r2 =  343 x 3 x 7 / 22 x 5

r2  = 7203/110 = 65.48

r = 8.09

Quiz Time

1. The radius of the cone is 5cm and the vertical height is 12cm. Find the area of the curved surface.

2. Find the curved surface area of a cone with base radius 5cm and the slant height 7cm.

3. Find the total surface area of a right circular cone, its slant height is 21cm, and the diameter of its base is 24cm.

4. A conical vessel whose internal radius is 5cm and height is 24cm is full of water. The water is poured into a cylindrical vessel with an internal radius of 7cm. Find the height to which the water rises.