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Thread: trigonometry

  1. #1
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    trigonometry

    You are using trigonometry very well in geometry. Iknow all the trig. formulas but I can only solve equations.
    I also don't like memorizing anything. I would like to learn if I can find sin18 or sin36 without memorizing anything or without referring to a trigonometry table.
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  3. #3
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    Re: trigonometry

    That seems alot of word. An equation of fifth degree. I know: 22.5,30,90,180,67.5,60,45.
    Butthey don't seem enough to calculate 36 degrees or 72 degrees.
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    Re: trigonometry

    Quote Originally Posted by kastamonu View Post
    That seems alot of word. An equation of fifth degree. I know: 22.5,30,90,180,67.5,60,45.
    Butthey don't seem enough to calculate 36 degrees or 72 degrees.
    ... and after factoring out an $x$, it became a fourth degree equation quadratic in $x^2$ ... did you even follow the derivation?

    Do you understand that $\dfrac{\pi}{5}$ and $36^\circ$ are the same angle?
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    Re: trigonometry

    I don't know this multiple-angle formula. I am seeing it for the first time. I am not responsible from it. It is out of the school curriculum.

    We just know cos(a+b),cos(2a),tan(a-b),sina+sinb,cosa+cosb.. etc....
    Last edited by kastamonu; May 26th 2017 at 01:38 PM.
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    Re: trigonometry

    It is easy to derive. The triple angle formula is found by $\sin(3x) = \sin(2x +x)$. Use the sum of angles formula. Then, for the 5x angle formula, use the sum of angles formula for $\sin(3x+2x)$. Easy peasy.
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  8. #8
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    Re: trigonometry

    many thanks.
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