# parabola

Also found in: Dictionary, Thesaurus, Acronyms, Wikipedia.

## parabola

**parabola** (pərăbˈələ), plane curve consisting of all points equidistant from a given fixed point (focus) and a given fixed line (directrix) (see illustration). It is the conic section cut by a plane parallel to one of the elements of the cone. The axis of a parabola is the line through the focus perpendicular to the directrix. The vertex is the point at which the axis intersects the curve. The *latus rectum* is the chord through the focus perpendicular to the axis. Examples of this curve are the path of a projectile and the shape of the cross section of a parallel beam reflector.

## parabola

(pă-**rab**-ŏ-lă) A type of conic section with an eccentricity equal to one. See also paraboloid.

*The Great Soviet Encyclopedia*(1979). It might be outdated or ideologically biased.

## Parabola

a curve that is the intersection of a circular cone by a plane parallel to a tangent plane to the cone (Figure 1, a); it thus is a conic section. A parabola can also be defined as the locus of points in a plane (Figure 1, b) such that each point is equidistant from a fixed point *F* of the plane and from a given line *MN; F* is called the focus and *MN* the directrix of the parabola. The line passing through the focus perpendicular to the directrix and directed from the directrix toward the focus is the axis of the parabola. The point at which the axis intersects the parabola is the vertex of the parabola.

Let us choose the coordinate system *xOy,* as shown in Figure 1, b. The equation of the parabola then takes the form

*y*^{2} = 2*px*

where *p* is the length of the segment *FN* and is called the parameter of the parabola. The parabola is a quadratic curve; it is the graph of the quadratic trinomial *y = ax ^{2} + bx + c*. It extends to infinity and is symmetric with respect to its axis.

If a light source is placed at the focus of a parabola, the rays reflected by the parabola will form a parallel beam, since the angle formed by the normal *PR* and the straight line *PF* connecting any point *P* of the parabola to the focus is equal to the angle that *PR* forms with a line parallel to the axis. This property of the parabola is used, for example, in projectors.

## parabola

[pə′rab·ə·lə]*y*=

*ax*

^{2}+

*bx*+

*c*.

## parabola

*y*

^{2}= 4

*ax*, where 2

*a*is the distance between focus and directrix