Page 4 - Lesson Notes-Ch.13 SA and Volumes(Cone)
P. 4
It is given that:
Height of the traffic cone (h) = 21 cm
Slant height of the traffic cone (l) = 28 cm
Now we know that,
2
2
2
l = r + h
2
2
2
28 = r + 21
2
2
2
r = 28 − 21
r = 7 7 cm
22 2
2
Area of the circular base = πr = ( 7 7 ) = 1078cm
2
7
2
Therefore, the area of the base is 1078 cm .
Example: 4
Find the total surface area of a right circular cone with radius 6 cm and height 8 cm.
Solution:
It is given that:
Radius of the cone (r) = 6 cm
Height of the cone (h) = 8 cm
Total Surface Area of the Cone (T. S. A) =?
2
2
= √ℎ + = √6 + 8 = √36 + 64
2
2
= √100 = 10
T. S. A = Curved Surface Area of Cone + Area of Circular Base
22 22
2
= rl + r = 6 10 + 6 6
7 7
+
1320 792
= 301.171 cm 2
2
=
=
301.171cm
7
2
Therefore, the area of the base is 301.71 cm .
Example: 5
The radius and slant height of a cone are in the ratio 4: 7. If its curved surface area is
2
792 cm , find its radius.
Solution:
It is given that
Curved surface area = πrl = 792
4
