Circles, Arcs and Sectors

The two formulas

• The distance round a circle is its circumference, and the space inside is its area
• Both formulas are printed in the List of formulas at the front of the paper
  • Area: A = πr²
  • Circumference: C = 2πr, which is the same as C = πd because the diameter is twice the radius
• Read the diagram carefully to see whether the number given is the radius or the diameter, since halving or doubling at the wrong moment is the usual error

• Area needs the radius squared, so a diameter must be halved first
• Circumference is a length and takes units such as cm; area is a space and takes squared units such as cm²

Answers in terms of π

• Questions frequently ask for the answer "in terms of π", which means leaving π in the answer rather than multiplying it out
• Work in exact numbers throughout and collect the π at the end, so a circle of radius 5 has area 25π cm² and circumference 10π cm
• "In its simplest form" alongside it means the number in front of π should be fully simplified
• Only round when the question actually asks for a decimal, and then to whatever accuracy it states