A circle is a plane shape formed by a curve such that all points on the curve are the same distance from a fixed point, called the centre.
Definitions
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A circumference is the distance around a circle.
A radius is the distance from the centre to any point on the circumference.
A diameter is a chord that passes through the centre.
The length of the diameter is twice the length of the radius.
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An arc is a part of the circumference.
A major arc is more than half of the circumferene.
A minor arc is less than half of the circumference.
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A chord is a line joining two points on the circumference.
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A segment is an area of a circle bounded by a chord and the circumference.
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A sector is an area of a circle bounded by two radii and the circumference.
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A tangent is a line that touches the circumference of the circle at only one point.
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Concyclic points lie on the circumference of the same circle.
e.g. A, B and C are concyclic points.
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Concentric circles have the same centre.
e.g. The two circles shown are concentric.
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Radii
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Two radii in a circle form part of an isosceles triangle.
e.g. OAB is an isosceles triangle.
 A = B
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Tangents
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A tangent to a circle forms an angle of 90°with
the radius at the point of contact.
e.g. OAB = 90°
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Two tangents to a circle from a point are equal in length.
A line from the point to the centre of the circle is an axis of symmetry.
e.g. AB = AC
m is an axis of symmetry.
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