Measuring Rotation

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Key Questions

  • Negative angles has to do with the direction of rotation that you consider in order to measure angles.

    Normally you start counting your angles from the positive side of the x axis in an anti-clockwise direction of rotation:

    You can also go clockwise and so to avoid confusion you use a negative sign to indicate this kind of rotation.

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  • You can draw an angle in standard position by positioning its vertex at the origin and one "ray" on the positive x-axis. The ray on the x-axis is called the initial side and the other ray is called the terminal side.
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    An angle is then measured POSITIVE for a counterclockwise rotation and NEGATIVE for a clockwise rotation:
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    When two angles have the same initial and terminal sides, they are said to be coterminal angles.
    Angles of −315° and 45° are coterminal angles.
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