National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Conic Sections and Parallel Li
This question and answer site from Dr. Math at the Math Forum looks at the situation of cutting a cone with a plane to get parallel lines. Can this be done?
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.