Theory
The radius is the distance from the centre of a circle to the edge.
The diameter is the distance all the way across the circle through the centre. Diameter = 2 x radius.
Circumference is the distance around the circle. Circumference = πd or 2πr.
Area is the space inside the circle. Area = πr².
The most common mistake is using diameter in the area formula. Area uses radius, not diameter.
For calculator questions, use the π button unless the question tells you to use a rounded value.
If the question gives diameter and asks for area, halve the diameter first to get the radius.
Circumference answers use length units such as cm or m. Area answers use square units such as cm² or m².
Do not round too early. Keep the calculator value until the final answer, then round to the requested accuracy.