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#1 2014-02-07 19:12:03

niharika_kumar
Member
From: Numeraland
Registered: 2013-02-12
Posts: 832

trigonometry#5

A vertical tower stands on a horizontal plane and is surmounted by a flagstaff of height 7 m. At a point on the plane, the angle of elevation of the bottom of the flagstaff is 36° and that of the top of the flagstaff is 45° .Find the height of the tower.

P.S:-I constructed a diagram but I am not able to deal with 36° without using trigonometric table. Pls. help.

View Image: trigo.gif

friendship is tan 90°.

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#2 2014-02-07 20:33:13

Agnishom
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From: The Complex Plane
Registered: 2011-01-29
Posts: 14,207
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Re: trigonometry#5

I think using a table is okay in this case


'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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#3 2014-02-07 23:00:03

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 81,414

Re: trigonometry#5

Hi;


In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

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#4 2014-02-07 23:44:18

bob bundy
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Registered: 2010-06-20
Posts: 6,092

Re: trigonometry#5

hi Niharika,

I'm sure tables (or maybe a calculator) is acceptable here.  Trying to get a value for tan 36 without,  is going to be a tough one.  eek

Bob


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

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#5 2014-02-08 01:11:41

Nehushtan
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From: London
Registered: 2013-03-09
Posts: 602
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Re: trigonometry#5


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#6 2014-02-08 01:32:03

Agnishom
Real Member
From: The Complex Plane
Registered: 2011-01-29
Posts: 14,207
Website

Re: trigonometry#5

We can do the nested use of this formula


'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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#7 2014-02-08 01:59:58

gAr
Member
Registered: 2011-01-09
Posts: 3,462

Re: trigonometry#5

Hi niharika_kumar,

Perhaps the simplest method is to use De Moivre's formula, taking x= pi/5

Expand LHS and equate the imaginary parts

We can then easily solve it as a quadratic, putting y=sin(x)^2:

Hence,


and

and


"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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#8 2014-02-08 02:07:57

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 81,414

Re: trigonometry#5

Hi;

The polynomial for cos(x/5) has a neat factorization so it is very easy.


In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

Online

#9 2014-02-08 02:15:00

gAr
Member
Registered: 2011-01-09
Posts: 3,462

Re: trigonometry#5

Hi,

Yes, that's also good.


"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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#10 2014-02-11 16:58:55

niharika_kumar
Member
From: Numeraland
Registered: 2013-02-12
Posts: 832

Re: trigonometry#5

thanks
got it now.
will do some more study on De Moivre's formula.


friendship is tan 90°.

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#11 2014-02-11 17:25:02

gAr
Member
Registered: 2011-01-09
Posts: 3,462

Re: trigonometry#5

You're welcome.


"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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