Radii of circles

Given a circle with center $O$ and a point $A$ on the circle, the segment $OA$ is called a radius. The length $r = OA$ is the radius of the circle. All radii of a circle are congruent. The diameter is twice the radius: $d = 2r$. The radius is used in formulas for the area ($A = \pi r^2$) and circumference ($C = 2\pi r$) of the circle.

Important properties

• All radii of a circle are equal in length.

• The diameter is twice the radius.

• The radius is perpendicular to the tangent at the point of contact.

• The radius is used in formulas for area and circumference.