## Quick Reference

A figure formed by the intersection of a plane and a cone. If the intersecting plane is perpendicular to the axis of a right circular cone, the figure formed is a circle. If the intersecting plane is inclined to the axis at an angle in excess of half the apex angle of the cone it is an ellipse. If the plane is parallel to the sloping side of the cone, the figure is a parabola. If the plane cuts both halves of the cone a hyperbola is formed.

A conic can be defined as a plane curve in which for all points on the curve the ratio of the distance from a fixed point (the **focus**) to the perpendicular distance from a straight line (the **directrix**) is a constant called the **eccentricity***e*. For a parabola *e* = 1, for an ellipse *e* < 1, and for a hyperbola *e* > 1.

**From:**
conic
in
A Dictionary of Physics »

*Subjects:*
Mathematics.

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