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Sin theta question

  1. Feb 10, 2010 #1
    1. The problem statement, all variables and given/known data

    sorry if the title is not that descriptive

    2. Relevant equations
    i is the imaginary number
    e is 2.7...
    x is any anlgle in radians that is... um see the example I explain it

    sin x = +/- i/2 (1/e^(ix) +/- e^(ix))

    I will use a example in atempt at a solution to show you how to use this equation

    3. The attempt at a solution

    the first +/- depends on were the angle is located on the unit circle

    the second +/- depends on if the angle is smaller or larger than pi/4
    - if it is smaller than pi/4 (by the way pi as in 3.14...) just simple enter your angle in the equation above and make the +/- a "-" in its radian measure
    - if it is larger than pi/4 take your angle and subtract pi/4 from it and use that as your x and use a "+" instead of minus in the equation

    - if anlge is greater than pi/2 use coresponding angle in the first quadrant if you want and just work out the "+/-" in the very begining of the equation


    I want to know what the sin of 60 degrees is... I know it is SQRT(3)/2 but i'll use the formula above for a demonstration ok...

    so 60 degrees in radians is pi/3 which is greater than pi/4 so I have to add pi/4 from pi/3... I'll do this in degrees sense it is easier... 60-45 = 15 degrees

    and now what I want to do is take that angle and do 45 minus that angle
    45-15 = 30 degrees = pi/6

    now just plug this into the equation above
    sin x = +/- i/2 (1/e^(ix) +/- e^(ix))
    sin x = + i/2 (1/e^((i pi)/6) + e^((i pi)/6) and the calculator gives .8660254038 i
    not really sure why it gives the i??? can someone answer that???
    if you remove the i this is the exact value for sin pi/3 or simple SQRT(3)/2

    try using the equation for other angles for example
    sin 1 degree = i/2(1/e^((i pi)/180) - e^((i pi)/180)) = .0174524064 which is the exact value of sin 1 degree

    MY QUESTION is how do I go backwards
    for example

    sin x = +/- i/2 (1/e^(ix) +/- e^(ix)) = SQRT(3)

    how do I solve for x???

  2. jcsd
  3. Feb 11, 2010 #2


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    Solving for x is as simple as taking the arcsine...are you trying to solve for x by exclusively using the complex exponential form of sine?

    See this article: http://en.wikipedia.org/wiki/Euler's_formula

    As far as I can tell, there should be no +/- in the equation, it should all be taken care of.
  4. Feb 12, 2010 #3
    yes i am trying to solve for x in that formula how do i do this?
  5. Feb 12, 2010 #4


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    You've made using Euler's formula way more complicated than it needs to be. Euler's formula is

    [tex]\sin x = \frac{e^{ix}-e^{-ix}}{2i}[/tex]

    for any value of x. You don't have plus or minuses depending on what the value of x is.

    To go backwards, let [itex]z=e^{ix}[/tex]. Then [itex]1/z = e^{-ix}[/itex], so the formula becomes:

    [tex]\sin x = \frac{z-\frac{1}{z}}{2i}[/tex]

    Solve for z. Once you have z, you can solve for x.
  6. Feb 12, 2010 #5
    I don't know were to go from here

    2i sin x = z - 1/z
  7. Feb 12, 2010 #6

    Gib Z

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    It is a quadratic equation in disguise.
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