**Angle - Wikipedia**
https://en.wikipedia.org/wiki/Angle

A reference angle is the acute version of any angle determined by repeatedly subtracting or adding straight angle (1 / 2 turn, 180°, or π radians), to the results as necessary, until the magnitude of the result is an acute angle, a value between 0 and 1 / 4 turn, 90°, or π / 2 radians. For example, an angle of 30 degrees has a reference ...

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