Formed when a plane slices a cone parallel to the slant side of the cone. It is the set of all points that are equidistant from a fixed line, the directrix, and a fixed point, the focus, which is not on the line. Another key feature of a parabola is the focal length, |p|, or distance from the vertex to the focus along the axis of symmetry. The direction of p is determined by the location of the focus in relation to the vertex. If p is positive, the parabola opens in the positive direction. If p is negative, the parabola opens in the negative direction. The focal width is the length of the line segment through the focus, perpendicular to the axis of symmetry.   

Khan Academy Practice:

• Writing Equation of Parabola

• Focus and Directrix