Angles in a circumference

  To represent angles in a circumference:
- we take the O (0,0) point as vertex
- we use the radius on the positive semiaxis of abscissas as origin
- we draw the extreme of the angle by measuring it:
        - counterclockwise, if it is positive
        - clockwise, if it is negative

  If the radius is 1, we call it goniometric circumference.

  We have four quadrants:


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