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    Hi,

    I don't understand the last lines of both part k) and part l) in the attached image. Why does the sign change?
    Shouldn't it remain positive, or am I missing something fundamental?

    I understand that it is Cos(5x--6x) + iSin(5x--6x), but shouldn't it become Cos(11x) + iSin(11x) rather than Cos(11x) - iSin(11x)?

    The text book answers has the same result as the image attached, too. What am I doing wrong?
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    (Original post by JLegion)
    Name:  ImageUploadedByStudent Room1468162156.769421.jpg
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    Hi,

    I don't understand the last lines of both part k) and part l) in the attached image. Why does the sign change?
    Shouldn't it remain positive, or am I missing something fundamental?

    I understand that it is Cos(5x--6x) + iSin(5x--6x), but shouldn't it become Cos(11x) + iSin(11x) rather than Cos(11x) - iSin(11x)?

    The text book answers has the same result as the image attached, too. What am I doing wrong?
    I wish I remembered to help you
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    The text book question was:
    (Cos5x + iSin5x) / (Cos3x - iSin3x)^2

    The first two lines in the attached image also appear to be mistakes as well.
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    The whole answer is wrong.
    (cos3x + isin3x)^2 != (cos(-3x) + isin(-3x))^2
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    Pretty sure the answers are wrong, both of them, their working out is really dodgy.

    Use the fact that cos(x)+isin(x)=e^{ix}

Cos(-x)=cos(x)

-sin(x)=sin(-x)

    And solve them using exponentials
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    Maybe its easier to write it in exponential form and you can just think of it as laws of indices.
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    (Original post by JLegion)
    Name:  ImageUploadedByStudent Room1468162156.769421.jpg
Views: 343
Size:  70.5 KB

    Hi,

    I don't understand the last lines of both part k) and part l) in the attached image. Why does the sign change?
    Shouldn't it remain positive, or am I missing something fundamental?

    I understand that it is Cos(5x--6x) + iSin(5x--6x), but shouldn't it become Cos(11x) + iSin(11x) rather than Cos(11x) - iSin(11x)?

    The text book answers has the same result as the image attached, too. What am I doing wrong?
    The edexcel textbook for each unit has so many mistakes it's actually a joke. One mistake I came across was that they couldn't even calculate tan49 or something like that. I stuck to past papers and madasmaths.
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    The book's working looks dodgy, I don't think it's right.

    \displaystyle z=\frac{e^{-i\theta }}{\left ( e^{-2i\theta } \right )^{3}}=e^{-i\theta -\left ( -6i\theta \right )} \iff z= \cos5\theta + i \sin 5\theta
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    I'm glad I'm not in the wrong. Normally I would assume the textbook was false, but due to both Edexcel & PhysicsAndMathsTutor getting both part k) and l) different to me, I naturally was very doubtful.
    I appreciate all the responses!
 
 
 
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