Circles

Core formulas:

Circumference = 2*pi*r
Area = pi*r^2
Arc length = theta/360 * 2*pi*r
Sector area = theta/360 * pi*r^2

Chord and tangent facts:

Perpendicular from center to chord bisects chord.
Tangents from same external point are equal.
Radius is perpendicular to tangent at point of contact.
Angle in a semicircle = 90 deg.
Equal chords subtend equal angles.

Cyclic quadrilateral:

Opposite angles sum to 180 deg.
Exterior angle equals opposite interior angle.

Circles​

Circles