Maths Challenge
Maths challenge.net: Bag of Balls
A "three star" geometry problem which requires prior math problem solving skills and proof writing skills to solve this "Bag of Balls" radius of balls problem.
Wolfram Research
Wolfram Math World: Sphere
This webpage thoroughly discusses the math of spheres. Includes the equation for a sphere in Cartesian and spherical coordinates as well as parametrically. Surface Area, volume, and curvature are addressed at this extensive site.
Houghton Mifflin Harcourt
Holt, Rinehart and Winston: Homework Help Independent Practice: Spheres
Get independent practice working with spheres. Each incorrect response gets a text box explanation and another try. Correct responses are confirmed.
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.
Calculator Soup
Calculator Soup: Sphere Calculator
This online calculator will calculate the 3 unknown values of a sphere given any 1 known variable including radius r, surface area A, volume V and circumference C. It will also give the answers for volume, surface area, and circumference...
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.