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# Help with C3 trig please? watch

1. Currently C3 trig is making me want to die because I've spent about an hour on about 3 questions. Here are the 3 I'm struggling with (hint, they all have sin2x in)
1) cos^2(x) - sin2(x) = sin^2(x)
2) 2sin2(x) = 3tan(x)
3) 5sin2(x) + 4sin(x) = 0

If anyone could help on any of them then I'd be really grateful.
2. I'll go one by one. For the first, assuming you mean , remember that and so . How does this help here?
3. (Original post by AxSirlotl)
Currently C3 trig is making me want to die because I've spent about an hour on about 3 questions. Here are the 3 I'm struggling with (hint, they all have sin2x in)
1) cos^2(x) - sin2(x) = sin^2(x)
2) 2sin2(x) = 3tan(x)
3) 5sin2(x) + 4sin(x) = 0

If anyone could help on any of them then I'd be really grateful.
for 1.)
cos^2(x)-sin^2(x)=sin(2x)
cos^2(x)-sin^2(x)=cos(2x) so

cos(2x)=sin(2x)
divide by cos(2x) but remember you get cos(2x)=0 as a lost solution.
4. (Original post by _gcx)
I'll go one by one. For the first, assuming you mean , remember that and so . How does this help here?
Is there a guide on how to type maths equations like that because I've been looking but I can't find one.
What I mean is cos squared x - sin 2x = sin squared x (I hope that makes more sense.
5. (Original post by AxSirlotl)
Is there a guide on how to type maths equations like that because I've been looking but I can't find one.
What I mean is cos squared x - sin 2x = sin squared x (I hope that makes more sense.
There's a guide here. From you have . Is there any identity that you've learnt that you can apply to the LHS here?
6. (Original post by _gcx)
I'll go one by one. For the first, assuming you mean
But... but... that's not an identity otherwise :/
7. (Original post by RDKGames)
But... but... that's not an identity otherwise :/
Did I say it was an identity?

Spoiler:
Show

I meant the double angle formula for the LHS
8. (Original post by RDKGames)
But... but... that's not an identity otherwise :/
It's not clear from the question, but I think that these are equations to be solved, rather than identities. OP - can you clarify?
9. (Original post by Pangol)
It's not clear from the question, but I think that these are equations to be solved, rather than identities. OP - can you clarify?
I think it's pretty safe to assume that they're equations to be solved.
10. (Original post by _gcx)
There's a guide here. From you have . Is there any identity that you've learnt that you can apply to the LHS here?
Thanks, should I get ?
11. (Original post by Pangol)
It's not clear from the question, but I think that these are equations to be solved, rather than identities. OP - can you clarify?
They need to be solved
12. (Original post by _gcx)
Did I say it was an identity?
(Original post by Pangol)
It's not clear from the question, but I think that these are equations to be solved, rather than identities. OP - can you clarify?
Eh yeah, they're equations to solve, I thought OP meant identities to prove without knowing he can use the equivalence relation in his post, then gcx just skipping the \equiv part by being lazy and use = instead

Most of these threads are identities so I just skipped to assume without reading the thread properly lol
13. (Original post by AxSirlotl)
Thanks, should I get ?
Easy mistake to make, , but . Think double angle formulas.
14. (Original post by _gcx)
Easy mistake to make, , but . Think double angle formulas.
Oh yeah I've just realised that it's the double angle formula.
15. (Original post by AxSirlotl)
Oh yeah I've just realised that it's the double angle formula.
So are you fine with saying we have ? Where can we go from there?

Btw, you can click on LaTeX here to see what code was used to produce it
16. (Original post by _gcx)
So are you fine with saying we have ? Where can we go from there?

Btw, you can click on LaTeX here to see what code was used to produce it
I got to and then from there got to which is a quadratic but I don't know if I've made a mistake somewhere or if I've done the right thing.
17. (Original post by AxSirlotl)
I got to and then from there got to which is a quadratic but I don't know if I've made a mistake somewhere or if I've done the right thing.
Not quite sure what you did there. You can just divide both sides by to get
18. (Original post by _gcx)
Not quite sure what you did there. You can just divide both sides by to get
I only managed to get one of the correct solutions from what I did, thanks for the help, I have a habit of over-complicating things.

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