This CoolMath resource provides an introduction to some Precalculus formulas.

Reinforce your knowledge of the circumference and the area of a circle with these activities. [Requires Java.]

This online calculator will find the area, circumference, and radii of an annulus. With any 2 known variables, you can calculate the other 5 unknowns.

Practice finding the radius and diameter of a circle using both vocabulary and visuals. Students receive immediate feedback and have the opportunity to try questions repeatedly, watch a video or receive hints.