Labels for the different parts of a circle are given below
Radius – the radius of a circle is the distance from the centre labelled O to the circumference. The radius is labelled |
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The circumference of a circle is the perimeter, and is labelled |
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Diameter – diameter of a circle is the length of a straight line between opposite ends drawn through the centre. The diameter is labelled |
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Tangent – A straight line drawn outside the circle which just touches it. |
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Chord - a line (not through the centre) drawn between any two points on the circumference. |
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Arc – a part of the circumference. The length of an arc is |
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Sector - the part of a a circle between two radii and an arc. We can divide a circle into major and minor sectors. The area of a sector is |
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Segment - the part of a a circle between chord and an arc. We can divide a circle into major and minor segments. The area of the minor segment on the right is |
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and is equal to
where
is the radius and
is the angle in radians. 
where
where