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# Double angle formulae trig equation watch

1. Solve sin4θ = cos2θ

What do you do with the sin4θ? I tried turning it into 2sin2θcos2θ using the double angle formulae but I don't know where to go from there...

Because say if I then did:

2(2sinθcosθ)(1-2sin2θ) = 1-2sin2θ

or...

2(2sinθcosθ)(2cos2θ-1) = wtf shall I use

I don't even know what to do- am I supposed to make everything so it's all in terms of sinθ or cosθ?? Nothing is seeming to work...
2. Absolute first thing you do: set , so your equation becomes . Solve for alpha, and then divide by 2 to find theta.
3. (Original post by DFranklin)
Absolute first thing you do: set , so your equation becomes . Solve for alpha, and then divide by 2 to find theta.
Where would you get α from?
4. (Original post by jessyjellytot14)
Where would you get α from?
It's just a substitution for the sake of simplicity, you can choose if you want, doesn't matter.

However, I do not see why you are expressing in a different form. Simply get everything on one side at stage, and just factor out a . You can then solve for theta.
5. (Original post by RDKGames)
Simply get everything on one side at stage, and just factor out a . You can then solve for theta.
Oooooh, that is the logical thing to do! How did I not think of that?

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