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Homework Help: Finding radian angle.

  1. Oct 25, 2016 #1
    1. The problem statement, all variables and given/known data
    For the value of cos θ, determine the radian value of θ if π ≤ θ ≤ 2π.

    Value is -1/2.

    2. Relevant equations

    3. The attempt at a solution
    I got the first value which is cos-1(-1/2) which I got (2π)/3.
    However I am unsure how to get the second value.
    I thought that since the first value is in second quadrant the other value is in the third quadrant, then I would take θ-π to find my value in the quadrant. But my answer was incorrect. Can someone pls guide me? Thx.
  2. jcsd
  3. Oct 25, 2016 #2


    Staff: Mentor

    What angle in the 3rd quadrant has a cosine of -1/2? Draw a unit circle and use basic geometry to find that angle.
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