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## #1 2014-04-18 16:35:54

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

### trigonometry#3

If

prove that:-

Last edited by niharika_kumar (2014-04-18 16:36:18)

friendship is tan 90°.

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## #2 2014-04-18 23:17:07

bob bundy
Registered: 2010-06-20
Posts: 8,113

### Re: trigonometry#3

hi Niharika,

Put tan = sin/cos and square the equation.

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 2014-04-19 19:08:02

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

### Re: trigonometry#3

I did that but couldn't succeed in proving the above.

friendship is tan 90°.

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## #4 2014-04-20 02:09:35

bob bundy
Registered: 2010-06-20
Posts: 8,113

### Re: trigonometry#3

hi Niharika,

I'm sorry; I thought this would come out easily.  Changing the tan is the standard thing I would do for one like this.  Then everything is in terms of sine and cos so it should have worked.

But I cannot do it either.  Below I have shown the graphs for (i) tan and root(sin - cos) and then for (ii) sin and cos cubed.

The first pair only cross in positive real numbers and twice in each 2pi repeat of the graphs.  The second pair cross once only in positive values in each 2pi repeat.  And nowhere near the same value of x (theta).  So it would seem that this cannot be shown for real numbers.  Odd.

Is it possible there's a 'typo' somewhere?

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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## #5 2014-04-20 02:30:07

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

### Re: trigonometry#3

The exact question is :-
If θ≠n*pi and tanθ is the geometric mean between sinθ and cosθ , then prove that 2-4sin^2θ+3sin^4θ-sin^6θ=1

geometric mean was given what I mentioned and the solution solved the above in just a single line saying

after that sinθ was taken in terms of cos^3θ and the question was solved easily.

But I couldn't get how this result was obtained..

Last edited by niharika_kumar (2014-04-20 02:31:36)

friendship is tan 90°.

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## #6 2014-04-20 02:57:04

bob bundy
Registered: 2010-06-20
Posts: 8,113

### Re: trigonometry#3

I think the geometric mean of a and b is

Now what I suggested will work.

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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## #7 2014-04-20 03:01:14

gourish
Member
Registered: 2013-05-28
Posts: 153

### Re: trigonometry#3

hi Niharika can you please use the latex code for your expression cause the question seems quite clear and i did get sinθ=cos^3 θ
use the G.M. formula if tanθ is the G.M. between sinθ and cosθ then tanθ=sqroot(sinθ*cosθ) then u easily get sinθ=cos^3 θ so just post the expression clearly so we can work it out

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #8 2014-04-20 03:02:01

bob bundy
Registered: 2010-06-20
Posts: 8,113

### Re: trigonometry#3

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 2014-04-20 03:02:57

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

### Re: trigonometry#3

may be the sign was put in by mistake.
thanks for the correction, the answer came out easily.:D
I spent 2 hrs around it and wasted so many sheets of paper, phew!!

Last edited by niharika_kumar (2014-04-20 03:03:37)

friendship is tan 90°.

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## #10 2014-04-20 03:12:25

bob bundy
Registered: 2010-06-20
Posts: 8,113

### Re: trigonometry#3

After half an hour I start suspecting an error and make the graphs.

Especially as I make sign mistakes all the time.

2-4sin^2θ+3sin^4θ-sin^6θ=1

Did you manage this bit?

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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## #11 2014-04-20 15:47:26

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

### Re: trigonometry#3

yes.
I did put the equation as

then I used the formula

and simplified it.

hence it was proved.

friendship is tan 90°.

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