Curated OER
Tangent Lines and the Radius of a Circle
Your Geometry learners will collaboratively prove that the tangent line of a circle is perpendicular to the radius of the circle. A deliberately sparse introduction allows for a variety of approaches to find a solution.
Varsity Tutors
Varsity Tutors: Hotmath: Practice Problems: Circles
Twenty-three problems present a variety of practice working with various aspects of circles. They are given with each step to the solution cleverly revealed one at a time. You can work each step of the problem then click the "View...
Purple Math
Purplemath: Completing the Square: Circle Equations
This tutorial works through two examples step by step that shows how to complete the square for finding the center and radius of a circle.
Wolfram Research
Wolfram Math World: Radius
This math site from MathWorld.com defines radius in terms of both a circle and a sphere. The site includes a diagram and links to related topics.
Math Words
Math words.com: Radius of a Circle or Sphere
This site from MathWords.com explains and shows visually what the radius of a sphere is.