Conic Sections - Parabola

Parabola is locus of all points which are equally spaced from a fixed line and a fixed point.

• The fixed point is called, focus of the parabola.
• Fixed line is called, directrix of the parabola.

In the above figure, F is the focus and line with points A, B and C is the directrix.

According to definition of parabola,

AM = MF
BN = NF
CO = OF

Axis of parabola is a line which is perpendicular to the directrix and passes through the focus of parabola. Vertex of a parabola is the point of intersection of axis and the parabola. Point O is the vertex of the parabola, as shown in the figure below.

Standard Equation

Consider parabola in the given figure,

The simplest form of the equation of a parabola is found when the vertex is at the origin in the coordinate plane.

Let point F(a,0) be focus and O(0,0) be the vertex of the parabola. A and B are two points on the directrix and point P(x,y) is any point on the parabola.

By definition of parabola, O is the midpoint of AF and O is a vertex of the parabola.

That is, AO = OF and coordinates of A will be (-a,0).

Therefore, the equation of the directrix is x+a=0.

Also, FP = AB, (by the definition of parabola)

Since,PB is perpendicular to directrix, coordinates of B is (-a,y)

Using distance formula we get,

This is equation for a parabola whose focus is at (a,0)where a > 0.

Equation of Parabola

Equation of the parabola for the figure given below is,

Equation of the parabola for the figure given below is,

Equation of the parabola for figure given below is,

The equations (1), (2), (3) and (4) are known as standard equations of a parabola.

Length of Latus rectum

Latus rectum of a parabola is the line segment perpendicular to the axis through focus and its endpoints lie on the parabola.

Length of the latus rectum of the parabola

AB is the latus rectum of the above parabola with focus F(a,0).

ABis perpendicular to the X- axis.

Perpendicular distance between the directrix and the focus is,

NF = 2a =AM

By definition of parabola,

AM = AF = 2a

Similarly,

FB = 2a

Therefore,

length of latus rectum = 2a + 2a = 4a

Video Lesson

Vertex and Directrix of Parabola

Parabola Examples

Example: Find equation of directrix, coordinates of focus and length of latus rectum of parabola \(\begin{array}{l}y^2 = 16x\end{array} \) Solution: Comparing given equation \(\begin{array}{l}y^2 = 16x\end{array} \) with \(\begin{array}{l}y^2 = 4ax\end{array} \) gives, 4a = 16,a = 4 Co-ordinate of Focus of the parabola is (4,0) We know, the Equation of directrix is x + a = 0 \(\begin{array}{l}\Rightarrow\end{array} \) x = – a Thus x = -4 Length of latus rectum is, 4a = 4 × 4 = 16 units

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