MEPX
Chapter 4 of 10All chapters
  1. 1Angles
  2. 2Triangles
  3. 3Area and perimeter
  4. 4Circles
  5. 5Similarity and scale
  6. 6Three dimensions
  7. 7Trigonometry
  8. 8Coordinate geometry
  9. 9Transformations
  10. 10Geometry in the world

Chapter 4 of 10

Circles

Pi, and where it appears.

The two formulas

Circumference is pi times the diameter. Area is pi times the radius squared. Mixing up radius and diameter is the most common mistake in the topic.

  • Diameter is twice the radius, so circumference is also 2 pi r.
  • An arc and a sector are simply fractions of the full circle.

Angle facts

The angle at the centre is twice the angle at the circumference on the same arc, and the angle in a semicircle is a right angle. Both fall out of the same symmetry.