National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Ellipse or Circle? Parametric Equation
Dr. Math, in reply to a student question, shows how a specific pair of parametric equations produces a circle as opposed to an ellipse.
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.