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## #1 2009-08-16 16:40:09

Fla\$h
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### Trigonometry Proofs

Hi Guys,i hope im posting in the right forum.Here Are A Few Exercises:-

PROVE THE FOLLOWING:-

1)

2)

3)

4)

5)

6)

I got these out of a maths textbook

## #2 2009-08-16 17:56:07

bobbym

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### Re: Trigonometry Proofs

Hi Fla\$h;

For #1)

Cross multiply and we have.

Last edited by bobbym (2009-08-16 18:04:19)

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.

## #3 2009-08-16 17:57:16

juriguen
Full Member

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### Re: Trigonometry Proofs

Hi!

I think I got the first one:

1.

Jose

“Make everything as simple as possible, but not simpler.” -- Albert Einstein

## #4 2009-08-16 18:04:50

Identity
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### Re: Trigonometry Proofs

Most trig proofs involve converting 'tan' and 'cot' to 'sin' and 'cos', and lots of fractions!

## #5 2009-08-16 18:24:00

yamini
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### Re: Trigonometry Proofs

hey i got ans for 4th ques
;)
sinA-2sincubeA/2coscubeA-cosA
take sinA common in numerator and cos A in denominator
we get
sinA(1-2sinsquareA)/cosA(2cossquareA-1)
cos2A formula is 1-2sinsquareA and 2cossquareA-1
so they both will get cancelled
we get
sinA/cosA=tanA
hence proved. :P:

## #6 2009-08-17 08:25:55

bobbym

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### Re: Trigonometry Proofs

Hi Fla\$h;
Here is another way for #4

Last edited by bobbym (2009-08-17 08:26:21)

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.

## #7 2009-08-17 10:50:25

bobbym

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### Re: Trigonometry Proofs

Hi Fla\$h;
for #6

Expand the LHS.

Last edited by bobbym (2009-08-17 10:56:20)

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.

## #8 2009-08-22 10:01:47

Fruityloop
Full Member

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### Re: Trigonometry Proofs

Hi Fla\$h!;
Here's #2...

using

we get

Last edited by Fruityloop (2009-08-22 10:11:56)

The eclipses from Algol (an eclipsing binary star) come further apart in time when the Earth is moving away from Algol and closer together in time when the Earth is moving towards Algol, thereby proving that the speed of light is variable and that Einstein's Special Theory of Relativity is wrong.

## #9 2009-08-22 12:40:11

bobbym

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### Re: Trigonometry Proofs

Hi Fla\$h;

Problem  #3 is not an identity.

Now here is the proof for #3:

Now cross multiply:

Now substitute cos²(A) = 1- sin²(A) and clean up:

Last edited by bobbym (2009-08-22 14:12:07)

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.

## #10 2009-08-22 17:45:12

bobbym

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### Re: Trigonometry Proofs

Hi Fla\$h;

For problem #5

Expanding the LHS and remembering that:

We get:

Now using:

and

We get:

Using:

Last edited by bobbym (2009-08-22 17:48:56)

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.