A chord is a line segment joining two points on the circle.
A diameter is a special chord that passes through the centre. A diameter is twice the length of the radius.
A tangent line (or simply tangent) to a curve is a straight line that ‘just touches’ the curve at a point without crossing the curve.
An arc is a part of the circumference.
A sector of a circle is two-dimensional shape enclosed by an arc and the two radii to its endpoints.
A semicircle is a sector whose arcs ends are the ends of a diameter.
The green part of the diagram is a semicircle Semicircle C A Arc Sector θ B
Tangent comes from the Latin tangents meaning ‘touching’, like in the word ‘tangible’.
Tangent O Diameter Chord
Parts of circles and their angles 360° 180° Semicircle Full circle Practice questions 16·1
1. O is the centre of the circle on the right.
O
Name each of the four diff erent coloured parts of the circle.
2. M is the centre each of the circles shown below. Name each of the outlines of the shaded regions in the circles as either a sector, semicircle, quarter circle or full circle.
(i) M (ii) M (iii) M (iv) M
3. Are the coloured outlines of the following shapes sectors, semicircles, quarter circles or circles?