Year 9

LLL - Measurement

Circles: Circumference and Area

A visual refresher on boundary, interior area, radius, circumference, and area of a circle.

Ready 0 / 60

Boundary first. Interior second. Formula last.

This lesson starts by separating the distance around a shape from the space inside it, then applies that idea to circles.

15boundary or area clicks
15radius clicks
30circle calculations
1question per page

Circumference or perimeter

distance around

C = 2 × π × r

Circumference is the perimeter of a circle.

Area of a circle

inside

A = π × r²

Area measures the space inside the circle.

Teacher refresher

Distance around vs space inside

Perimeter and circumference live on the boundary. Area is the inside surface.

Perimeter or circumference

perimeter circumference
Look at the outline. The answer lives on the edge.

Area

inside inside
Look at the surface. The answer is the region inside the boundary.
Perimeteraround a polygon
Circumferencearound a circle
Areaspace inside
Radiuscentre to edge

Circle formulas

Radius drives both formulas

The radius is the distance from the centre to the circumference. The diameter is twice the radius.

Circumference of a circle

r C = 2 × π × r
Circumference is the distance around the circle.

Area of a circle

r A = π × r²
Area is the space inside the circle.

Diameter connection

d d = 2r
If you are given diameter, halve it to get radius for area.

Calculation convention

Use π = 3.14

Round numerical answers to two decimal places. The diagram on each question tells you whether you have radius or diameter.

Circumference uses a length unit. Area uses a square unit.
Section Question

Question

Answer this page before moving on.

Question 0 / 0

Answer

Use the diagram.

Complete

Circle refresher finished

You have separated boundary from area, identified radius, and calculated circumference and area from diagrams.

0 / 15boundary or area
0 / 15radius
0 / 30calculations
0total attempts
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