Vol. 5 · What a circle keeps fixed

Circles

A companion to 《圆》 by Ke Xinli · 11 stops ·

Vol. 1Vol. 2Vol. 3Vol. 4Vol. 5Vol. 6Vol. 7Vol. 8

  1. Ch. 1Perimeter and Area of Circles, Sectors and SegmentsDon't grind out a shaded area. Move one piece and it often forms a whole shape.ConceptM7
  2. Ch. 2Symmetry of the CircleFold a circle on any diameter and it fits itself. That fold proves the theorem.ConceptM8
  3. Ch. 3Angles in a CircleAn inscribed angle stays put as its vertex moves; the central angle is twice it.ConceptM10
  4. Ch. 4Points, Lines and Circles: Relative PositionsPositions look like pictures, but you only compare two numbers, d and the radii.Concept
  5. Ch. 5Tangents to a CircleA tangent is a secant whose two points have merged; it meets the radius at 90°.Concept
  6. Ch. 6Concyclic PointsProve four points share a circle, and every equal angle on it is yours to use.MethodM10
  7. Ch. 7The Power of a PointAny line through a fixed point cuts the circle twice; PA·PB never changes.ConceptM9M11
  8. Ch. 8Power of a Point: ApplicationsEach point has a power OP² − r². Equal power for two circles: the radical axis.SkillM10
  9. Ch. 9Ptolemy's Theorem and Its ApplicationsOpposite-side products add up to ≥ the diagonal product; equal iff concyclic.ConceptM9M22
  10. Ch. 10Circumcenter and Incenter of a TriangleThe circumcenter serves the vertices, the incenter the sides; Euler links them.ConceptM11
  11. Ch. 11Miscellaneous Circle ProblemsMixed problems test if you spot the hidden equal angles, products or lengths.Method