Aptitude - Volume and Surface Area - Discussion

8. 

The slant height of a right circular cone is 10 m and its height is 8 m. Find the area of its curved surface.

[A]. 30 m2
[B]. 40 m2
[C]. 60 m2
[D]. 80 m2

Answer: Option C

Explanation:

l = 10 m,

h = 8 m.

So, r = l2 - h2 = (10)2 - 82 = 6 m.

Curved surface area = rl = ( x 6 x 10) m2 = 60 m2.


Viveka Shahi said: (Sep 5, 2014)  
But R is not radius. It is diameter of the right circular cone.

So answer should be [A].

Sunil said: (Nov 19, 2015)  
Curved surface area of cone = rl.

r stands for radius.
l stands for slant height.

r = ?
height = 8 m.
l = 10 m.

First we should find to r (radius). Then we can find curved surface area of cone.

r = l - r.
r = 10-8 = 6 m.

Put value of r in given formula.

rl = 6 m*10 m = 60 m^2.

(C) 60 m^2 correct option.

Subhasis said: (Feb 24, 2016)  
I think r should be 6/2 = 3.

What is the right answer?

Islam said: (Mar 7, 2016)  
C is correct.

l = 10 m,

h = 8 m.

So, r = l2 - h2 = (10)2 - 82 = 6 m.

Curved surface area = rl = ( x 6 x 10) m2 = 60 m2.

Krutarth Vaidya said: (Jun 22, 2016)  
C is also correct.

L = 10,

H = 8,

So, r = l2 - h2 = (10)2 - 82 = 6 m.

Area = 60 m^2.

Gowtham said: (Jan 15, 2017)  
@Sunil.

How 10-8 = 6?

It should be 2.

Justin said: (Feb 25, 2017)  
Answer is A, why you are taking as a full height of 10?

We need a radius only,
I think;
(22/7) x 6 x 5,
(22/7) x 30 => is a correct Answer.

Mustafa said: (Mar 30, 2017)  
Yeah, 10-8 should be 2 not 6.

Nikita said: (Apr 1, 2017)  
L = 10 m.
H = 8 m.
R = (10 * 10) - (8 * 8).
R = 100 - 64 = 36.
Square root of 36 is 6.
= 6 * 10 = 60m.

Ngawang Chojey said: (May 15, 2018)  
@Nikita is right.

You can use the formula of a right triangle to derive the length of base i.e. equal to radius: h^2=a^2+b^2.

Where, h=slant height, a=hight of a right triangle and b=base of a right triangle.

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