Radius: A line segment drawn joining the centre and the boundary line (known as circumference) is called the Radius of the circle.

Diameter: A line segment crossing the circle passing through the centre of the circle.

Circumference: The boundary or the perimeter of the circle.

Chord: A line segment joining any two points on the circumference of the circle.

Arc: Part of the circumference of the circle.

Tangent: A line that touches the circumference of the circle at exactly one point.

Sector: An area enclosed by two radii of a circle.

Basic formulae of a circle:

circumference = \(2*pi*r\), where \(r\) is the radius of the circle.

area = \(pi*r^2\)

diameter = \(2*r\)

Properties of Arcs and angles :

The angle made by the ends of the arc at the centre of the circle is called its angle.

- Equal arcs subtend equal angles at the centre of the circle and vice versa.
- The angle at the centre od the circle is twice the angle at the circumference subtended by the same arc.

Properties of Chords and tangents:

- Diameter is the longest chord of a circle.
- A perpendicular line from the centre to the chord bisects the chord.
- Equal chords are equidistant from the centre of the circle.
- Equal chords subtend equal angles at the centre of the circle.

- A tangent to a circle is perpendicular to the radius drawn at the point of contact.
- Tangents to a circle from an exterior point are equal.
- The angle between the tangent and the chord is equal to the inscribed angle on the opposite side of the chord.

Basic properties of a circle

- Angle in a semicircle is the right angle.
- Angle in the same segment of a circle are equal.

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Sandy

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