Circles

These are the parts of a circle.

circumference - the perimeter (outside length) of a circle

diameter - a straight line going through the centre of a circle from one side to another

radius - a straight line from the centre to the circumference (half the diameter)

sector - the space inside two radii ("radii" is the plural of radius)

chord - a straight line from one side of a circle to another

segment - the space between a chord and the circumference

tangent - a straight line that touches the circumference at only one point



Circumference and Area

There are two formulae involving π that you should know for your exam:

Circumference = 2πr   
(2 x π x r), r = radius

Area = πr^2
(π x r x r)

Note: π is a specific number used in calculating lengths and areas in circles. You do not need to remember the exact number (π = 3.14159...), but you do need to remember where the π button is on your calculator, and how to use it.

Question

Find the circumference and area of this circle

Radius, r = 4

Circumference = 2πr
= 2 x π x 4 = 8 x π
= 25.1 cm  (3 significant figures)

Area = πr^2
= π x 4 x 4 = 16 x π
= 50.3 cm^2  (3s.f.)



What's Next?
Circle Theorems involving working out angles.

MH