7th Grade · Geometry
Radius
7.G.B.4
Definition
The distance from the center of a circle to any point on the circle. All radii of a circle are equal. r = d/2. Used in circumference (C = 2πr) and area (A = πr²).
Examples
- If r = 5cm, C = 2π(5) = 31.4 cm.
- A = π(5)² = 25π ≈ 78.5 cm².
Key rule
The radius is half the diameter — most circle formulas use r.
Memory hook
Center to edge — that's the radius.