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#1 2013-10-11 19:15:25

niharika_kumar
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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°.

#2 2013-10-11 23:03:26

bob bundy
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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.  sad

Bob


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#3 2013-10-12 09:57:43

anonimnystefy
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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 (2013-10-12 10:01:21)


The limit operator is just an excuse for doing something you know you can't.
“It's the subject that nobody knows anything about that we can all talk about!” ― Richard Feynman
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment

#4 2013-10-12 16:18:09

niharika_kumar
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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°.

#5 2013-10-12 19:16:38

bob bundy
Moderator

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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


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

#6 2013-10-12 23:53:44

niharika_kumar
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Re: slant height of cone

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


friendship is tan 90°.

#7 2013-10-13 01:44:33

anonimnystefy
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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?


The limit operator is just an excuse for doing something you know you can't.
“It's the subject that nobody knows anything about that we can all talk about!” ― Richard Feynman
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment

#8 2013-10-13 17:58:17

bob bundy
Moderator

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Re: slant height of cone

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

Who knows?  dunno

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

#9 2013-10-13 19:58:43

anonimnystefy
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Re: slant height of cone

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


The limit operator is just an excuse for doing something you know you can't.
“It's the subject that nobody knows anything about that we can all talk about!” ― Richard Feynman
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment

#10 2013-10-14 18:34:11

niharika_kumar
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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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