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Maths question help - trigonometry

Given that (sin θ + cos θ ) 0, find all the solutions of
[2 cos 2θ(sin 2θ - √3 cos 2θ)] / (sin θ + cos θ) = √6 (sin 2 θ √3 cos 2 θ )
for 0 θ < 360°.

Please help, I'm not getting too far with this! :colondollar:
Thank you :smile:

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Reply 1
Original post by Katherine1996
Given that (sin θ + cos θ ) 0, find all the solutions of
[2 cos 2θ(sin 2θ - √3 cos 2θ)] / (sin θ + cos θ) = √6 (sin 2 θ √3 cos 2 θ )
for 0 θ < 360°.

Please help, I'm not getting too far with this! :colondollar:
Thank you :smile:


firstly divide the bracket (sin 2 θ √3 cos 2 θ ) and get solutions by R transformation

the remaining equation on tidy up will produce a difference of squares (cosθ + sinθ)(cosθ - sinθ)

can you see what to do next?
Reply 2
Original post by Katherine1996
Given that (sin θ + cos θ ) 0, find all the solutions of
[2 cos 2θ(sin 2θ - √3 cos 2θ)] / (sin θ + cos θ) = √6 (sin 2 θ √3 cos 2 θ )
for 0 θ < 360°.

Please help, I'm not getting too far with this! :colondollar:
Thank you :smile:


What a horrible looking equation - where did this come from? :biggrin:

OK, to start with note that both sides have a common factor sin 2 θ √3 cos 2 θ so EITHER this is 0 OR 2 cos / (sin θ + cos θ) = √6 (by dividing out that common factor).

Can you make some progress from here?
Original post by Katherine1996
Given that (sin θ + cos θ ) 0, find all the solutions of
[2 cos 2θ(sin 2θ - √3 cos 2θ)] / (sin θ + cos θ) = √6 (sin 2 θ √3 cos 2 θ )
for 0 θ < 360°.

Please help, I'm not getting too far with this! :colondollar:
Thank you :smile:


If you had 2abc=6b\dfrac{2ab}{c}=\sqrt6b

Then you could rearrange to get

2ab=6bc2ab = \sqrt6bc

Giving

2ab6bc=02ab - \sqrt6bc = 0

so

b(2a6c)=0b(2a-\sqrt6c) = 0

So that b=0b=0 or 2a6c=02a-\sqrt6c = 0


Can you take the same steps with your question and then complete to find the answers
Original post by TeeEm
firstly divide the bracket (sin 2 θ √3 cos 2 θ ) and get solutions by R transformation

the remaining equation on tidy up will produce a difference of squares (cosθ + sinθ)(cosθ - sinθ)

can you see what to do next?


I can find tan2θ = √3
but I'm confused on the other solutions :frown:
Original post by davros
What a horrible looking equation - where did this come from? :biggrin:

OK, to start with note that both sides have a common factor sin 2 θ √3 cos 2 θ so EITHER this is 0 OR 2 cos / (sin θ + cos θ) = √6 (by dividing out that common factor).

Can you make some progress from here?


It is AEA :wink:
I can do tan2θ = √3 , but I'm still confused on how I get solutions from 2 cos / (sin θ + cos θ) = √6 :-/
Reply 6
Original post by Katherine1996
I can find tan2θ = √3
but I'm confused on the other solutions :frown:


can you see cos2θ = cos2θ -sin2θ which is a difference of squares which cancels your bottom (as it is not equal to zero)?
Reply 7
Original post by Katherine1996
It is AEA :wink:
I can do tan2θ = √3 , but I'm still confused on how I get solutions from 2 cos / (sin θ + cos θ) = √6 :-/


Do as TeeEm suggested:

multiply both sides by sin θ + cos θ first.

Then write out the standard identity for cos and use the fact that this is a difference of two squares.

Note the common factor on both sides of your new equation and the fact you are given that a certain expression is not zero so you can divide by it.

Solve what is left :smile:
Original post by davros
Do as TeeEm suggested:

multiply both sides by sin θ + cos θ first.

Then write out the standard identity for cos and use the fact that this is a difference of two squares.

Note the common factor on both sides of your new equation and the fact you are given that a certain expression is not zero so you can divide by it.

Solve what is left :smile:


To solve it, do I need to use Rcos(θ+α) form?
Reply 9
Original post by Katherine1996
To solve it, do I need to use Rcos(θ+α) form?


If you've ended up with something like cos A - sin A = something

then yes - you can probably virtually write down what R and alpha are going to be :smile:
Original post by davros
If you've ended up with something like cos A - sin A = something

then yes - you can probably virtually write down what R and alpha are going to be :smile:


Ah it's not working :frown: any help?
Reply 11
Original post by Katherine1996
Ah it's not working :frown: any help?


What equation have you ended up with and how are you trying to solve it?

I'm a bit tired to be trying this sort of thing now, so if someone doesn't help you see it through tonight I'll be back on tomorrow, but you're probably only a step or two away from getting to the end now :smile:
Original post by davros
What equation have you ended up with and how are you trying to solve it?

I'm a bit tired to be trying this sort of thing now, so if someone doesn't help you see it through tonight I'll be back on tomorrow, but you're probably only a step or two away from getting to the end now :smile:


I am not sure why there is any harmonic form going on?
Original post by davros
What equation have you ended up with and how are you trying to solve it?

I'm a bit tired to be trying this sort of thing now, so if someone doesn't help you see it through tonight I'll be back on tomorrow, but you're probably only a step or two away from getting to the end now :smile:


I've got to θ+α = 30
but I think I've missed somewhere what α is

It's fine if you can't do it now :smile: I'm sure I'll figure it out, thanks for your help!
Reply 14
Original post by TenOfThem
I am not sure why there is any harmonic form going on?


I've only been skimming this online instead of writing it out properly, but it looks like it's going to end up with solving something like

cos A - sin A = something

at the end unless I've missed something in the working.

So it's about 1 step away from being able to write down the solution.
Original post by davros
I've only been skimming this online instead of writing it out properly, but it looks like it's going to end up with solving something like

cos A - sin A = something

at the end unless I've missed something in the working.

So it's about 1 step away from being able to write down the solution.


You are correct - I had = 0 but I had not done any working out
Reply 16
Original post by Katherine1996
I've got to θ+α = 30
but I think I've missed somewhere what α is

It's fine if you can't do it now :smile: I'm sure I'll figure it out, thanks for your help!


You basically want to be thinking about cos(X + 30) or some multiple of it (X = theta cos I'm lazy) :smile:
Original post by davros
You basically want to be thinking about cos(X + 30) or some multiple of it (X = theta cos I'm lazy) :smile:


Okay, thank you :smile: I think I can have a mess around with this last bit now, and hopefully come to the answers :smile:
Reply 18
Original post by Katherine1996
Okay, thank you :smile: I think I can have a mess around with this last bit now, and hopefully come to the answers :smile:


Sorry - I think I mean to write cos(X + 45) not 30.

(I was tired and wrote down rubbish and then didn't realize till after I'd logged off!!)
where is this question from?

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