Theory:

A circle is a two-dimensional figure with no corners and edges. It is a simple closed curve but not a polygon.
Parts of the circle:
Centre
 
The fixed point in which the circle is described is called the centre of the circle.
 
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Radius:
 
The line segment joining any point on the circle with its centre is the radius of the circle.
 
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Chord:
 
A line segment joining two points on a circle is called a chord.
 
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Diameter:
 
The chord of a circle passing through the centre is called the diameter of the circle. It is twice the length of the radius.
 
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Circumference:
 
The distance around the circle is its circumference.
 
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Arc:
 
Any portion of the circumference of a circle is called an arc. It is denoted using the symbol ""
 
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Sector:
 
The plane surface enclosed between two radii and the circular arc of a circle is called a sector of a circle.
 
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Segment:
 
The region enclosed by the chord and an arc is called the segment of the circle.
 
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Semicircle:
 
The diameter of the circle divides into two semi-circles. Thus, the semicircle is half of the circle.  
 
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Interior of the Circle:
 
The area within the boundary of the circle.
 
Exterior of the Circle:
 
The area outside the boundary of the circle.
 
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