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    I'm trying to solve a C3 trig question involving trig identities and whilst two of my solutions are correct, I'm missing a solution and the model answer completes the problem in a completely different way. Can anyone tell me where I'm going wrong? Thanks.

    The question and my answer
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    Whereas the actual answer
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    (and gets the same solutions as me as well as 90 degrees)
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    You can't cancel the cos in your 3rd line of working. That takes out a solution
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    (Original post by ryanroks1)
    ...
    If I had the equation x^2 = 2x, do you agree that it would be folly of us to cancel an x from both sides and get x = 2? Surely you know that what you really need to do is x^2 - 2x = 0 \Rightarrow x(x-2) = 0 which gets you two solutions.

    The same applies with your trigonometric equation. Do not cancel the \cos, you should bring it over to the other side and factorise it out whereby you can say that another set of solutions is given by \cos \theta = 0.
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    (Original post by Zacken)
    If I had the equation x^2 = 2x, do you agree that it would be folly of us to cancel an x from both sides and get x = 2? Surely you know that what you really need to do is x^2 - 2x = 0 \Rightarrow x(x-2) = 0 which gets you two solutions.

    The same applies with your trigonometric equation. Do not cancel the \cos, you should bring it over to the other side and factorise it out whereby you can say that another set of solutions is given by \cos \theta = 0.
    Great explanation as usual!

    Just a side note - would you say we cannot cancel the cos because it's actually equal to 0 in one solution, and so dividing by 0 is not possible?

    Or is what I'm saying rubbish and only applicable to this example?
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    (Original post by Student403)
    Or is what I'm saying rubbish and only applicable to this example?
    Nope, that's precisely why. :yep:
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    (Original post by Zacken)
    Nope, that's precisely why. :yep:
    Wow that's really good to know - thank you
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    (Original post by Student403)
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    (Original post by Zacken)
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    Thank you, seems obvious now!
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    (Original post by ryanroks1)
    Thank you, seems obvious now!
    Nice going :five:
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    (Original post by Student403)
    You can't cancel the cos in your 3rd line of working. That takes out a solution
    Why are you giving advice when its incorrect?
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    (Original post by GeologyMaths)
    Why are you giving advice when its incorrect?
    Why are you acting condescending when you haven't yet stated what the "correct" advice would be?
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    (Original post by Student403)
    Why are you acting condescending when you haven't yet stated what the "correct" advice would be?
    correct advise is to minus cos then factorise
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    (Original post by GeologyMaths)
    correct advise is to minus cos then factorise
    OP asked "Where am I going wrong?", which I clearly answered in my post
 
 
 
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