Khan Academy
Khan Academy: Praxis Core Math: Circles Lesson
A basic review of circles and how to calculate the circumference, radius, diameter, and area. Includes practice questions.
Math Open Reference
Math Open Reference: Radius of a Circle
Learn about the radius of a circle with this useful reference guide. Provides a detailed definition, examples, and an interactive activity that enhances further explanation. Includes links to related topics.
National Council of Teachers of Mathematics
The Math Forum: Pi Day
Need some ideas for Pi Day on March 14th? You can get some great resources at this location. There are all kinds of links available.