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circular measure

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A method of measuring angles by treating them as the angle formed by a sector of a circle at the circle's centre. The unit of measure is the radian, the angle subtended at the centre of a circle by an arc of equal length to the radius. Since an arc of length r subtends an angle of 1 radian, the whole circumference, length 2πr, will subtend an angle of 2πr/r = 2π radians. Thus, 360° = 2π radians; 1 radian = 57.296°.

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