Circles: Parts
Radius, diameter, chord, tangent, secant, arc and sector.
Key idea
Radius, diameter, chord, tangent, secant, arc and sector.
In depth
A circle is all points at a fixed distance (radius) from a centre. Diameter = 2r. A chord joins two points on the circle, a tangent touches at exactly one point, a secant crosses at two, and an arc is a piece of the circle.
Worked example
Radius 5 gives diameter 10. The longest chord is the diameter.
Real-world uses
- Naming parts of wheels, pizzas and gears
- Describing a circular track or a circular table
Common mistake
Confusing radius and diameter in formulas.
How to learn this
- Read the key idea and say it in your own words.
- Work through the worked example, then try a similar problem.
- Use the interactive lesson’s visual and mini-quiz to check the pattern.
Frequently asked questions
What is Circles: Parts?
Radius, diameter, chord, tangent, secant, arc and sector. It is a topic at the beginner level in Geometry.
Where is Circles: Parts used in real life?
Real-world examples: Naming parts of wheels, pizzas and gears; Describing a circular track or a circular table.
What should I learn before Circles: Parts?
Make sure you are comfortable with Regular Polygon Area, the previous topic in Geometry.
What is a common mistake with Circles: Parts?
Confusing radius and diameter in formulas.
How can I practise Circles: Parts?
Open the interactive lesson, play with the visual, then answer the three-question mini-quiz. Getting two right marks the topic complete.
What comes after Circles: Parts?
Move on to Circumference next.
Related beginner topics in Geometry
- Points, Lines and Planes – The undefined building blocks of geometry.
- Line Segments and Rays – A segment has two endpoints; a ray starts at a point and goes on forever.
- Angles – Two rays sharing an endpoint; measured in degrees.
- Types of Angles – Acute <90°, right 90°, obtuse between 90° and 180°, straight 180°.
- Measuring Angles with a Protractor – Align the baseline and read the scale.
- Complementary and Supplementary – Pairs summing to 90° or 180°.