Radius, diameter, chord, tangent, arc, sector and segment, and why two radii always make an isosceles triangle.
What you will learn
- Name the parts of a circle
- Use radius and diameter
- Use the isosceles triangle made by two radii
The names
- A radius is a straight line from the centre to the circle. A diameter is a chord that passes through the centre, so it is two radii long.
- A chord is a straight line joining two points on the circle. A tangent is a straight line that touches the circle at exactly one point.
- The circumference is the distance all the way round the circle. An arc is part of the circumference. A sector is a region bounded by two radii and an arc (a slice of pizza). A segment is a region bounded by a chord and an arc.
The radius is half the diameter: a circle of diameter 9 cm has radius 4.5 cm. A circle that fits exactly inside a square, touching all four sides, has the diameter as the side of the square: diameter 9 cm gives a square of perimeter 4 × 9 = 36 cm.
The rest of this lesson and its 6 practice questions are locked.
In this lesson
- The names
- Two radii make an isosceles triangle
- Try it: First question
6 practice questions, marked as you go; about 15 minutes in all.
Key terms
Radius, Diameter, Chord, Tangent, Circumference, Arc, Sector, Segment
In this unit: Circles and circle theorems
The parts of a circle, the circle theorems and how to give them as reasons, tangents, chains of reasoning, and the equation of a circle centred at the origin with its tangents.