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    Solve the equation for 0 < theta< 360

    Sin 2theta-tan theta=0

    is it best to use the double angle formula to expand first?
    and would this give me

    2sintheta cos theta- tan theta=0

    where would I go from here because I have three signs :confused:
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    Change tan to sin/cos as well
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    As a general principle, I would change everything into signs and cosines of theta
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    multiply by cos, then divide by sine
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    i swear thats not C3!!!! Edexcel???
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    (Original post by 2strong)
    i swear thats not C3!!!! Edexcel???
    yes it is. it's pretty simple too.
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    I can't believe I missed changing the tan into sin/cos

    but then again I'm sort of new to this. thanks
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    (Original post by milan5baros)
    I can't believe I missed changing the tan into sin/cos

    but then again I'm sort of new to this. thanks
    when you get used to it, you'll do this sort of thing automatically, without having to write it out.
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    after some manipulation I get too sin theta(1-2sinsquared theta)

    and from this I get the reference angle 45degrees, and then 135 degrees using CAST.

    the book has about six answers, can anyone spot where I may have gone wrong here.
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    \sin\theta=0, \theta=...
    \sin\theta=\pm\frac{1}{\sqrt2},\  theta=...

    Does the question restrict theta to a certain range, because otherwise there are infinitely many values of theta which satisfy the equation
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    Yes, there is a range requirement. (0,360)

    Although I have ended up with different solutions
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    Just saw it.

    I got \sin\theta(2\cos^2\theta-1)=0 myself
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    x = theta

    sin2x - tanx = 0
    (2sinxcosx) - (sinx/cosx) = 0
    2sinxcosx = sinx/cosx
    2sinxcos^2x = sinx
    2sinx(1 - sin^2x) = sinx

    now just expand the brackets on t RHS and you'll be fine from there.

    NB. I haven't done any maths since my about june so sorry if i'm wrong
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    Please don't put too much of a solution, hints are usually more helpfully, although what identities do we know involving 2 \cos ^2 x -1
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    cos^2x = 1 - sin^2x
 
 
 
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Updated: August 27, 2008

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