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# C2 Trigonometrical identities

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1. C2 Trigonometrical identities

2sin²x = 3sinxcosx + 2cos²x

(0< X< 2pi) Answers to 3 sig.fig.

2. Re: C2 Trigonometrical identities
use the sin^2+cos^2=1 rule and see were you can collect and cancel like terms eventually you will get one trigonometric sign and then that will equal a number.
3. Re: C2 Trigonometrical identities
(Original post by mind)
use the sin^2+cos^2=1 rule and see were you can collect and cancel like terms eventually you will get one trigonometric sign and then that will equal a number.
In that case how would you get rid of the sinxcosx in the middle? I think this is more like factorizing into a double bracket with sinx in one, and cosx in the other.
4. Re: C2 Trigonometrical identities
Is it tanx=-2/3?
5. Re: C2 Trigonometrical identities
(Original post by torakrubik)
In that case how would you get rid of the sinxcosx in the middle? I think this is more like factorizing into a double bracket with sinx in one, and cosx in the other.
remember sinx/cosx= tanx

so probably divide everything by cosx?

wait no that won't work....let me try on paper for you
Last edited by nickss; 22-04-2012 at 13:23.
6. Re: C2 Trigonometrical identities
(Original post by torakrubik)
In that case how would you get rid of the sinxcosx in the middle? I think this is more like factorizing into a double bracket with sinx in one, and cosx in the other.
My vote is for factorising with a multiple of sin x at the beginning of each bracket and a multiple of cos x at the end.
7. Re: C2 Trigonometrical identities
Doesn't look like C2 to me. I suspect there is a typo in the question.

The best method would be use knowledge of double angle identities to find an equation
8. Re: C2 Trigonometrical identities
(Original post by tiny hobbit)
My vote is for factorising with a multiple of sin x at the beginning of each bracket and a multiple of cos x at the end.
How many C2 students do you think could factorise this? I would plump for 2%!
9. Re: C2 Trigonometrical identities
(Original post by Mr M)
Doesn't look like C2 to me. I suspect there is a typo in the question.

The best method would be use knowledge of double angle identities to find an equation
This was my first thought, but I resisted it because of the claim that the question was a C2 question.
10. Re: C2 Trigonometrical identities
(Original post by Mr M)
Doesn't look like C2 to me. I suspect there is a typo in the question.

The best method would be use knowledge of double angle identities to find an equation
Thank God for that then!! I can't do it and I am doing Add Maths FSMQ in year 11, which pretty much covers C2 Trigonometry.

Heard of this double angle identity before; never known what it was!
11. Re: C2 Trigonometrical identities
Divide everything by cos^2 x
Last edited by ReTurd; 22-04-2012 at 13:39.
12. Re: C2 Trigonometrical identities
(Original post by ReTurd)
Divide everything by cos^2 x
It is C2 then because once you have done that it is very easy!!

Cheers mate! You had the ingenuity to see this where no one else could!!
13. Re: C2 Trigonometrical identities
Tanx = 2, -1/2
14. Re: C2 Trigonometrical identities
x=243.4,63.4,333.4,153.4
15. Re: C2 Trigonometrical identities
So we have three possible approaches:

Last edited by Mr M; 22-04-2012 at 15:04. Reason: Fixed missing 2
16. Re: C2 Trigonometrical identities
(Original post by GreenLantern1)
x=243.4,63.4,333.4,153.4
You need to work in radians.
17. Re: C2 Trigonometrical identities
(Original post by Mr M)
So we have three possible approaches:

The first one would obviously be the C2 approach then.
18. Re: C2 Trigonometrical identities
(Original post by Mr M)
You need to work in radians.
Why?

Nothing wrong with degrees. Even if it is 0<x<2pi you can still work in degrees.
19. Re: C2 Trigonometrical identities
(Original post by GreenLantern1)
The first one would obviously be the C2 approach then.
Yes I think so. The second one wouldn't be impossible for a bright student. Anyone resitting C2 in their A2 year would almost certainly use the third approach.
20. Re: C2 Trigonometrical identities
(Original post by GreenLantern1)
Why?

Nothing wrong with degrees. Even if it is 0<x<2pi you can still work in degrees.
If you want to become a better mathematician, you really need to do something about that arrogance.

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Last updated: April 22, 2012
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