Circles

The equation

(x − h)² + (y − k)² = r² is a circle centred at (h, k) with radius r. Note it is r², not r, on the right.

Tangents

A tangent meets the circle once and is perpendicular to the radius at that point.

Arcs and sectors

A fraction of the whole. Arc length is (θ/360) × 2πr and sector area is (θ/360) × πr².