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# Don't understand trig proof Watch

1. Hi ppl,

There are two proofs which I cannot seem to follow through.

sin(x) = 2tan(x/2)/1+tan^2(x/2)

cos(x) = 1-tan^2(x/2)/1+tan^(x/2)

Some detailed derivation will be appreciated.

Thanks

Edit: Figured out cos(x)
2. How far do you manage to get? it may help if you post where you get to and how you got there, then we can say what point you either went wrong or what you should try next.
3. (Original post by Lakura225)
How far do you manage to get? it may help if you post where you get to and how you got there, then we can say what point you either went wrong or what you should try next.
For sinx, I don't know where to start it.

For cosx, I don't know if I have done a valid method but did figure it out after posting.

cosx = cos^(x/2) - sin^2(x/2) <= cos(x/2 +x/2)

= 1/sec^(x/2) - sin^2(x/2)

Making the same denominator: = 1 - tan^2(x/2)/1+tan^(x/2)
4. (Original post by mathsRus)
For sinx, I don't know where to start it.

For cosx, I don't know if I have done a valid method but did figure it out after posting.

cosx = cos^(x/2) - sin^2(x/2) <= cos(x/2 +x/2)

= 1/sec^(x/2) - sin^2(x/2)

Making the same denominator: = 1 - tan^2(x/2)/1+tan^(x/2)
look at Q 246 on the link

(around 400 trig questions with solutions)

(Apologies that the booklet is currently under construction, I was physically working on it when I saw your message.)
5. This site might help you when you get stuck, i got lazy and couldn't be bothered to find my identities :P

http://www.symbolab.com/solver/trigo...?origin=button
6. (Original post by TeeEm)
look at Q 246 on the link

(around 400 trig questions with solutions)

(Apologies that the booklet is currently under construction, I was physically working on it when I saw your message.)
Thanks again, I have test on Monday so hopefully I will go over what I can.

Also what do those (****) mean?
7. (Original post by mathsRus)

Also what do those (****) mean?
difficulty 4 star.

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