Radius (r)

Definition

The radius is the distance from the center of a circle or sphere to any point on its circumference or surface.

Imagine you have a pizza and you want to measure how far it is from the center to the crust. That distance is like the radius of the pizza.

Diameter: The diameter is twice the length of the radius, and it goes straight through the center of a circle or sphere.

Circumference: The circumference is the distance around a circle, which can be found by multiplying 2π by the radius.

Surface Area: The surface area of a sphere can be calculated using 4π times the square of its radius.

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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.