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## #1 2013-10-10 20:15:25

niharika_kumar
Member
From: Numeraland
Registered: 2013-02-12
Posts: 1,062

### slant height of cone

this ques. is based upon vol. and surface area.
the ques is an MCQ one.

Q.slant height of the cone of which the frustum is a part ,is equal to

options:-

a) hr1/(r1-r2)

b) lr1/(r1-r2)

c) (h+l)r1/(r1-r2)

d) h(r1+r2)/(r1-r2)

please help me out along with the process to solve this.

friendship is tan 90°.

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## #2 2013-10-11 00:03:26

bob bundy
Administrator
Registered: 2010-06-20
Posts: 8,137

### Re: slant height of cone

hi niharika_kumar

This is a bit tricky without the diagram or any explanation of what each letter stands for.

Below I've posted a diagram with some distances marked.  Can you do the same?

You may be able to work with similar triangles.

Let's say that the top cone has a height x, a slant height y and a radius r2 and for the whole cone, height x+h, slant height y + L, radius r1.

Then

and

I cannot match any of these answers yet so I need more information if I'm to give you any more help.

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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## #3 2013-10-11 10:57:43

anonimnystefy
Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

### Re: slant height of cone

Hi

If Bob's labels are correct I'd go for a).

Bob: You are using the wrong triangles. Use the big one and the small one with base of r1-r2 and height equal to the height of the frustum.

Last edited by anonimnystefy (2013-10-11 11:01:21)

Here lies the reader who will never open this book. He is forever dead.
Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

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## #4 2013-10-11 17:18:09

niharika_kumar
Member
From: Numeraland
Registered: 2013-02-12
Posts: 1,062

### Re: slant height of cone

thnx bob and anonimystefy but there is nothing extra info. given apart frm what i mentioned above . And i tried but couldn't figure it out.
all i can say is that in the book the answer is given as option b). pls see if you can bring it out using some methods .

friendship is tan 90°.

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## #5 2013-10-11 20:16:38

bob bundy
Administrator
Registered: 2010-06-20
Posts: 8,137

### Re: slant height of cone

hi niharika_kumar

There must be information to say what all those letters stand for.  Is there a diagram as part of an earlier question?

I'm sure that this question requires the use of similar triangles.  That's the way to get a formula involving the distances from the whole cone and the cone that is chopped off to make a frustrum.

I can get to

using my second diagram.  That looks quite a bit like answer b except r2 rather than r1.  But for r1 - r2 to be positive r1 must be the radius of the larger base, so I'm still puzzled.

anonimnystefy:

It comes to the same thing; either way you get an expression with r1 - r2.

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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## #6 2013-10-12 00:53:44

niharika_kumar
Member
From: Numeraland
Registered: 2013-02-12
Posts: 1,062

### Re: slant height of cone

i completely agree with u bob , i think the options printed there are wrong.
thnk u for helping

friendship is tan 90°.

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## #7 2013-10-12 02:44:33

anonimnystefy
Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

### Re: slant height of cone

bob bundy wrote:

hi niharika_kumar

There must be information to say what all those letters stand for.  Is there a diagram as part of an earlier question?

I'm sure that this question requires the use of similar triangles.  That's the way to get a formula involving the distances from the whole cone and the cone that is chopped off to make a frustrum.

I can get to

using my second diagram.  That looks quite a bit like answer b except r2 rather than r1.  But for r1 - r2 to be positive r1 must be the radius of the larger base, so I'm still puzzled.

anonimnystefy:

It comes to the same thing; either way you get an expression with r1 - r2.

Bob

Aren't we looking for L+y, not just y?

Here lies the reader who will never open this book. He is forever dead.
Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

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## #8 2013-10-12 18:58:17

bob bundy
Administrator
Registered: 2010-06-20
Posts: 8,137

### Re: slant height of cone

Aren't we looking for L+y, not just y?

Who knows?

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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## #9 2013-10-12 20:58:43

anonimnystefy
Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

### Re: slant height of cone

Well, L+y is the slant height of the whole cone, right?

Here lies the reader who will never open this book. He is forever dead.
Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

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## #10 2013-10-13 19:34:11

niharika_kumar
Member
From: Numeraland
Registered: 2013-02-12
Posts: 1,062

### Re: slant height of cone

anonimnystefy wrote:

Well, L+y is the slant height of the whole cone, right?

as per bob's diagram, YES.

friendship is tan 90°.

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