Texas Instruments
Texas Instruments: Circle Equations
Students will practice their knowledge of circle equations in the y= form by matching equations with pictures of various circles.
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.
National Council of Teachers of Mathematics
The Math Forum: Circles
Various forms of equations of circles are presented at the Math Forum: center-radius form, general form, diameter form, three-point form, and parametric form.
BBC
Bbc: The Circle
The equation of a circle is the focus of this site. Topics include tangents, intersecting circles, and intersection of lines and circles. Examples and a practice test are available.