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Directrix of Parabola

Directrix of parabola is perpendicular to the axis of the parabola. The directrix of the parabola is helpful in defining the parabola. A parabola represents the locus of a point that is equidistant from a fixed point called the focus, and the fixed-line called the directrix. The eqution of the directrix of parabola depends on the equation of the parabola, the focus, and the axis of the parabola.

Let us learn more about the directrix of parabola, and how to find the directrix of parabola.

1.	What Is Directrix of Parabola?
2.	How to Locate Directrix of Parabola?
3.	Uses of Directrix of Parabola
4.	Examples on Directrix of Parabola
5.	Practice Questions
6.	FAQs on Directrix of Parabola

What Is Directrix of Parabola?

The directrix of a parabola is a line that is perpendicular to the axis of the parabola. The directrix of the parabola helps in defining the parabola. A parabola represents the locus of a point which is equidistant from a fixed point called the focus and the fixed line called the directrix. The directrix and the focus are equidistant from the vertex of the parabola. Here we define the directrix for the standard equations of a parabola.

The directrix of the parabola y² = 4ax, having x-axis as its axis, passes through (-a, 0), and has the equation x + a = 0.
The directrix of the parabola y² = -4ax, having x-axis, passes through (a, 0), and has the equation x - a = 0.
The directrix of the parabola x² = 4ay, having y-axis as its axis, passes through (0, -a), and has the equation y + a = 0.
The focus of the parabola x² = -4ay, having y-axis as its axis, passes through (0, a), and has the equation y - a = 0.

Directrix of Parabola

How to Locate Directrix of Parabola?

The directrix of a parabola is a line perpendicular to the axis of the parabola. The directrix of a parabola lies at a distance of 'a' units from the vertex of the parabola. The directrix and the focus lie at equal distance from the vertex of the parabola and the equation of directrix can be calculated based on the equation of the parabola.

The parabola having an equation with a second degree in x has the y-axis or a line parallel to the y-axis as its axis, and the parabola having an equation with second degree in y had the x-axis or a line parallel to the x-axis as its axis.

Equation of a Parabola	Axis of the Parabola	Vertex of the Parabola	Directrix of the Parabola
(x - h)² = 4a(y - k)	x = h	(h, k)	y = k - a
(y - k)² = 4(x - h)	y = k	(h, k)	x = h - a

Uses of Directrix of Parabola

The directrix of a parabola is used to find the numerous features of the parabola.

The directrix of a parabola helps to write the equation of a parabola.
The directrix of a parabola helps to locate the axis of the parabola.
The directrix of the parabola is useful to find the equations of the focal chords.
The directrix of the parabola is useful to find the equation of the latus rectum and the endpoints of the latus rectum.

Examples on Directrix of Parabola

Example 1: Find the equation of a parabola having the directrix of parabola as x + 5 = 0, the x-axis as the axis of the parabola, and the origin as the vertex of the parabola.

Solution:

The directrix of parabola is x + 5 = 0. The focus of the parabola is (a, 0) = (5, 0).

For the parabola having the x-axis as the axis and the origin as the vertex, the equation of the parabola is y² = 4ax.

Hence the equation of the parabola is y² = 4(5)x, or y² = 20x.

Therefore, the equation of the parabola is y² = 20x.
Example 2: Find the directrix of parabola, having the equation (y - 7)² = 12(x - 4).

Solution:

The given equation of the parabola is (y - 7)² = 12(x - 4). The equation resembles the equation of the parabola (y - k)² = 4a(x - h).

The vertex is (h, k) = (4, 7), and 4a = 12, and a = 3.

Hence the focus is (h + a, k ) = (4 + 3, 7) = (7, 7).

The directrix is passing through (h - a, k) = (4 - 3, 7) = (1, 7)

The equation of the directrix of parabola is x = 1, or x - 1 = 0

Therefore, the equation of directrix of parabola is x - 1 = 0.

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Practice Questions on Directrix of Parabola

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FAQs on Directrix of Parabola

How Do I Find Directrix of a Parabola?

The directrix of a parabola can be found, by knowing the axis of the parabola, and the vertex of the parabola. For an equation of the parabola in standard form y² = 4ax, with focus at (a, 0), axis as the x-axis, the equation of the directrix of this parabola is x + a = 0 . Similarly, we can easily find the directrix of the parabola for the other forms of equations of a parabola.

What Is the Equation For Directricx of Parabola?

The equation of directrix is the line that is perpendicular to the axis of the parabola. For the given equation of the parabola we first need to find the vertex, focus, and axis of the parabola, to find the equation of directrix of the parabola. For a parabola of the form (x - h)² = 4a(y - k), the line parallel to the y-axis is the axis of the parabola, the vertex is (h, k), focus of parabola is (h, k + a), and the equation of directrix of parabola is y = k - a

How Are the Focus of Parabola, and Directrix of Parabola Related?

The focus of parabola is a point, and the directrix of the parabola is a straight line, which is together helpful to define the equation of a parabola. A parabola is the locus of a point which is equidistant from a fixed point called the focus, and the fixed-line called the directrix. The focus and the directrix lie on either side of the vertex of the parabola and are equidistant from the vertex.

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