Circle Basics and Key Formulae
Every circle has eight important parts you need to recognise. The radius goes from the centre to the edge, whilst the diameter cuts straight across through the centre (and equals 2 × radius). A chord is any straight line connecting two points on the circle, and a tangent touches the circle at exactly one point.
The circumference is the distance around the outside, like measuring a bike tyre. A sector looks like a pizza slice, an arc is just the curved edge of that slice, and a segment is the leftover bit when you draw a chord across a sector.
There are five essential formulae for circles: circumference (πd or 2πr), area of a circle (πr²), area of a sector , arc length , and perimeter of a sector (arc length + 2r). Remember that θ (theta) represents the angle at the centre in degrees.
Quick Tip: πr² sounds like "area" - use this to remember it's the area formula! For circumference, just remember πd.
