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Homework Help: Finding xy coordinates of obtuse and acute triangle

  1. May 11, 2012 #1
    This might seem easy, but im sort of rusty on the math since i havent taken a math course in a while.
    1. The problem statement, all variables and given/known data
    A 2 meter long bar lies in the xy plane with one end at the origin. find position at the xy plane of the other?

    end point of the bar if the angle the bar makes with the x axis is the followin
    a) 2pi/3
    b) 120 degree
    c) 30 degree

    2. Relevant equations

    3. The attempt at a solution
    do i use x=2cosx y=2sinx ? for the 30 degree angle one i used
    but when i get to 120degrees i used the sin law but somethin seems off
  2. jcsd
  3. May 11, 2012 #2


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    Homework Helper

    For the 120o and [itex]2\pi / 3 [/itex] angles, they're obtuse (between 90o and 180o) so you need to look into the second quadrant (this means the cos(x) value will be negative, while the sin(x) value will still be positive, but you need to reconsider what the value of x is because you're now dealing with a triangle that isn't using the angle you started with).

    Here's an illustration, notice [tex]\frac{\pi}{3}=\pi-\frac{2\pi}{3}[/tex]
    Which in degrees is equivalent to [itex]60^o=180^o-120^o[/itex]

    http://img835.imageshack.us/img835/6619/trianglehelp.png [Broken]
    Last edited by a moderator: May 6, 2017
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