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**niharika_kumar****Member**- From: Numeraland
- Registered: 2013-02-12
- Posts: 1,021

pls help

Q. Two plane mirrors A and B are aligned parallel to each other as shown in fig.

A light is incident at an angle 30° at a point just inside one end of A .The plane of incidence coincides with the plane of figure.Find the maximum number of times the ray undergoes reflections before it emerges out.

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,522

Do you have the picture?

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 86,481

I thought you had figured out how to post a picture?

**In mathematics, you don't understand things. You just get used to them.Of course that result can be rigorously obtained, but who cares?Combinatorics is Algebra and Algebra is Combinatorics.**

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Alternately you can upload the picture in a familiar site and post the image hotlink here

'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'

'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'

'Humanity is still kept intact. It remains within.' -Alokananda

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**niharika_kumar****Member**- From: Numeraland
- Registered: 2013-02-12
- Posts: 1,021

pls copy paste the following link for the fig.:-

http://www.mathisfunforum.com/viewtopic.php?pid=290049#p290049

post #77

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**niharika_kumar****Member**- From: Numeraland
- Registered: 2013-02-12
- Posts: 1,021

i am still not able to find the upload option in this section.

friendship is tan 90°.

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 86,481

It is in post reply on the bottom, I can make a picture of it if you need it.

**In mathematics, you don't understand things. You just get used to them.Of course that result can be rigorously obtained, but who cares?Combinatorics is Algebra and Algebra is Combinatorics.**

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'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'

'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'

'Humanity is still kept intact. It remains within.' -Alokananda

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 86,481

Hi;

The formula says 30.

**In mathematics, you don't understand things. You just get used to them.Of course that result can be rigorously obtained, but who cares?Combinatorics is Algebra and Algebra is Combinatorics.**

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**ShivamS****Member**- Registered: 2011-02-07
- Posts: 3,531

What a coincidence! This is the same question I encountered while taking the IIT JEE some years back. The answer is 30.

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,389

hi niharika_kumar

When the ray hits mirror B for the first time it has travelled in the right/left direction

This can happen until it has travelled 2root 3 in that direction. Say it happens n times then

But the ray has also hit A at the start so add one, but 'just' misses B at the end so take one off.

Hence the maximum number of reflections is 30.

Bob

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

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**niharika_kumar****Member**- From: Numeraland
- Registered: 2013-02-12
- Posts: 1,021

thank you!

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**ShivamS****Member**- Registered: 2011-02-07
- Posts: 3,531

Niharika, you are preparing for the JEE?

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**niharika_kumar****Member**- From: Numeraland
- Registered: 2013-02-12
- Posts: 1,021

JEE is my goal and its preparations are going in parallel, but now I am in class10 so my focus is more on boards.

Did you give JEE,if yes then how was it?

friendship is tan 90°.

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What is JEE? Joint Entrance Examinations?

'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'

'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'

'Humanity is still kept intact. It remains within.' -Alokananda

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**niharika_kumar****Member**- From: Numeraland
- Registered: 2013-02-12
- Posts: 1,021

yes (as far as I know).

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**ShivamS****Member**- Registered: 2011-02-07
- Posts: 3,531

niharika_kumar wrote:

JEE is my goal and its preparations are going in parallel, but now I am in class10 so my focus is more on boards.

Did you give JEE,if yes then how was it?

Just saw this post right now...

It wasn't extremely hard. I managed to crack the main and the advanced (AIR 52) but decided to go to MIT instead. For your preparation, try plancess.com (it gives tutorials by IIT JEE crackers AIR <100). Use NCERT books for all the subjects, but for physics go for HC Verma.

Good luck.

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**niharika_kumar****Member**- From: Numeraland
- Registered: 2013-02-12
- Posts: 1,021

wow, that's really great.

thanks for your suggestion.will try that after my exams are over.

friendship is tan 90°.

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