The Basics (Leaving Cert Mathematics): Revision Notes

Circle Terminology

Circle Terminology

Explanation

A circle is a set of points in a plane that are equidistant from a fixed point called the centre. The terminology associated with circles is crucial for understanding their properties, relationships, and theorems in geometry. Below are key terms related to circles and their definitions.

Key Terms

• Arc: A part of the circumference of a circle between two points.
• Centre: The fixed point from which all points on the circle are equidistant.
• Chord: A line segment joining two points on the circumference of a circle.
• Circumference: The total distance around the circle.

• Diameter: A chord that passes through the centre of the circle; it is the longest chord.
• Radius: A line segment from the centre of the circle to any point on its circumference.
• Tangent: A line that touches the circle at exactly one point.
• Sector: A region of the circle enclosed by two radii and the arc between them.

• Segment: A region of the circle enclosed by a chord and the arc between the chord's endpoints.
• Semicircle: An arc of a circle whose endpoints are the endpoints of a diameter, dividing the circle into two equal halves.
• Circumcircle: A circle that passes through all the vertices of a polygon (e.g., a triangle).
• Incircle: A circle that fits inside a polygon and is tangent to all its sides.
• Concentric Circles: Two or more circles that share the same centre but have different radii.
• Point of Contact: The single point where a tangent touches a circle.
• Inscribed Angle: An angle formed by two chords in a circle sharing a common endpoint.
• Central Angle: An angle whose vertex is the centre of the circle and whose sides are radii.
• Minor Arc: An arc smaller than a semicircle.
• Major Arc: An arc larger than a semicircle.