Stage 18 · Circles

18.1.1 A Circle Is Born: Always the Same Distance from One Point

18.1 What Is a Circle — Definition and the Secret of Symmetry

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18.1.1 A Circle Is Born: Always the Same Distance from One Point

Core idea

Pin one end of a string, swing the other end all the way around, and every point you reach sits exactly as far from the pin as every other — that is a circle, written as circle O with radius r.

Module goal. Start from the one core idea — every point the same distance from a fixed center — to understand the circle, then unearth its most powerful weapon, symmetry, laying the foundation for every theorem to come.
eastmath.com · 18.1 What Is a Circle — Definition and the Secret of Symmetry · 18.1.1 A Circle Is Born: Always the Same Distance from One Point