The Trigonometry of Circles
Definitions and formulas for the arc and the arc length of a circle, sector and the area of the sector of a circle, the unit circle, the angles on the unit circle in radians, the angles on the unit circle in degrees, the points on the circumference of the unit circle
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arc of a circle  arc length of a circle  sector of a circle 
area of a sector of a circle  the unit circle  points on the circumference of the unit circle 
An arc of a circle:  
An arc of a circle is a segment of the circumference of the circle.  
The formula for the arc length of a circle:  
Arc length of a circle in radians: Arc Length = Arc length of a circle in degrees: Arc Length = 

A sector of a circle:  
A sector of a circle is a pie shaped portion of the area of the circle. Technically, the piece of pie is between two segments coming out of the center of the circle.  
The area of a sector of a circle:  
Sector area of a circle in radians: Sector Area = Sector area of a circle in degrees: Sector Area = 
The unit circle:  
The main angles of the unit circle in radians:  The main angles of the unit circle in degrees: 