How to find the equation of a parabola in standard form

A parabola is a graph of a quadratic function. Pascal stated that a parabola is a projection of a circle. Galileo explained that projectiles falling under the effect of uniform gravity follow a path called a parabolic path. Many physical motions of bodies follow a curvilinear path which is in the shape of a parabola. In mathematics, any plane curve which is mirror-symmetrical and usually is of approximately U shape is called a parabola. Here we shall aim at understanding the derivation of the standard formula of a parabola, the different standard forms of a parabola, and the properties of a parabola.

What is Parabola?

A parabola refers to an equation of a curve, such that a point on the curve is equidistant from a fixed point, and a fixed line. The fixed point is called the focus of the parabola, and the fixed line is called the directrix of the parabola. Also, an important point to note is that the fixed point does not lie on the fixed line. A locus of any point which is equidistant from a given point (focus) and a given line (directrix) is called a parabola. Parabola is an important curve of the conic sections of the coordinate geometry.

Parabola Equation

The general equation of a parabola is: y = a(x-h)2 + k or x = a(y-k)2 +h, where (h,k) denotes the vertex. The standard equation of a regular parabola is y2 = 4ax.

Some of the important terms below are helpful to understand the features and parts of a parabola.

• Focus: The point (a, 0) is the focus of the parabola
• Directrix: The line drawn parallel to the y-axis and passing through the point (-a, 0) is the directrix of the parabola. The directrix is perpendicular to the axis of the parabola.
• Focal Chord: The focal chord of a parabola is the chord passing through the focus of the parabola. The focal chord cuts the parabola at two distinct points.
• Focal Distance: The distance of a point \((x_1, y_1)\) on the parabola, from the focus, is the focal distance. The focal distance is also equal to the perpendicular distance of this point from the directrix.
• Latus Rectum: It is the focal chord that is perpendicular to the axis of the parabola and is passing through the focus of the parabola. The length of the latus rectum is taken as LL' = 4a. The endpoints of the latus rectum are (a, 2a), (a, -2a).
• Eccentricity: (e = 1). It is the ratio of the distance of a point from the focus, to the distance of the point from the directrix. The eccentricity of a parabola is equal to 1.

Standard Equations of a Parabola

There are four standard equations of a parabola. The four standard forms are based on the axis and the orientation of the parabola. The transverse axis and the conjugate axis of each of these parabolas are different. The below image presents the four standard equations and forms of the parabola.

The following are the observations made from the standard form of equations:

• Parabola is symmetric with respect to its axis. If the equation has the term with y2, then the axis of symmetry is along the x-axis and if the equation has the term with x2, then the axis of symmetry is along the y-axis.
• When the axis of symmetry is along the x-axis, the parabola opens to the right if the coefficient of the x is positive and opens to the left if the coefficient of x is negative.
• When the axis of symmetry is along the y-axis, the parabola opens upwards if the coefficient of y is positive and opens downwards if the coefficient of y is negative.

Parabola Formula

Parabola Formula helps in representing the general form of the parabolic path in the plane. The following are the formulas that are used to get the parameters of a parabola.

• The direction of the parabola is determined by the value of a.
• Vertex = (h,k), where h = -b/2a and k = f(h)
• Latus Rectum = 4a
• Focus: (h, k+ (1/4a))
• Directrix: y = k - 1/4a

Graph of a Parabola

Consider an equation y = 3x2 - 6x + 5. For this parabola, a = 3 , b = -6 and c = 5. Here is the graph of the given quadratic equation, which is a parabola.

Direction: Here a is positive, and so the parabola opens up.

Vertex: (h,k)

h = -b/2a

= 6/(2 ×3) = 1

k = f(h)

= f(1) = 3(1)2 - 6 (1) + 5 = 2

Thus vertex is (1,2)

Latus Rectum = 4a = 4 × 3 =12

Focus: (h, k+ 1/4a) = (1,25/12)

Axis of symmetry is x =1

Directrix: y = k-1/4a

y = 2 - 1/12 ⇒ y - 23/12 = 0

Derivation of Parabola Equation

Let us consider a point P with coordinates (x, y) on the parabola. As per the definition of a parabola, the distance of this point from the focus F is equal to the distance of this point P from the Directrix. Here we consider a point B on the directrix, and the perpendicular distance PB is taken for calculations.

As per this definition of the eccentricity of the parabola, we have PF = PB (Since e = PF/PB = 1)

The coordinates of the focus is F(a,0) and we can use the coordinate distance formula to find its distance from P(x, y)

PF = \(\sqrt{(x - a)^2 + (y - 0)^2}\)
= \(\sqrt{(x - a)^2 + y^2}\)

The equation of the directtrix is x + a = 0 and we use the perpendicular distance formula to find PB.

PB = \(\frac{x + a}{\sqrt{1^2 + 0^2}}\)

=\(\sqrt{(x + a)^2}\)

We need to derive the equation of parabola using PF = PB

\(\sqrt{(x - a)^2 + y^2}\) = \(\sqrt{(x + a)^2}\)

Squaring the equation on both sides,

(x - a)2 + y2 = (x + a)2

x2 + a2 - 2ax + y2 = x2 + a2 + 2ax

y2 - 2ax = 2ax

y2 = 4ax

Now we have successfully derived the standard equation of a parabola.

Similarly, we can derive the equations of the parabolas as:

• (b): y2 = – 4ax,
• (c): x2 = 4ay,
• (d): x2 = – 4ay.

The above four equations are the Standard Equations of Parabolas.

Properties of a Parabola

Here we shall aim at understanding some of the important properties and terms related to a parabola.

Tangent: The tangent is a line touching the parabola. The equation of a tangent to the parabola y2 = 4ax at the point of contact \((x_1, y_1)\) is \(yy_1 = 2a(x + x_1)\).

Normal: The line drawn perpendicular to tangent and passing through the point of contact and the focus of the parabola is called the normal. For a parabola y2 = 4ax, the equation of the normal passing through the point \((x_1, y_1)\) and having a slope of m = -y1/2a, the equation of the normal is \((y - y_1) = \dfrac{-y_1}{2a}(x - x_1)\)

Chord of Contact: The chord drawn to joining the point of contact of the tangents drawn from an external point to the parabola is called the chord of contact. For a point \((x_1, y_1)\) outside the parabola, the equation of the chord of contact is \(yy_1 = 2x(x + x_1)\).

Pole and Polar: For a point lying outside the parabola, the locus of the points of intersection of the tangents, draw at the ends of the chords, drawn from this point is called the polar. And this referred point is called the pole. For a pole having the coordinates \((x_1, y_1)\), for a parabola y2 =4ax, the equation of the polar is \(yy_1 = 2x(x + x_1)\).

Parametric Coordinates: The parametric coordinates of the equation of a parabola y2 = 4ax are (at2, 2at). The parametric coordinates represent all the points on the parabola.

☛ Also Check:

• Ellipse
• Hyperbola

FAQs on Parabola

What is Parabola in Conic Section?

Parabola is an important curve of the conic section. It is the locus of a point that is equidistant from a fixed point, called the focus, and the fixed-line is called the directrix. Many of the motions in the physical world follow a parabolic path. Hence learning the properties and applications of a parabola is the foundation for physicists.

What is the Equation of Parabola?

The standard equation of a parabola is y2 = 4ax. The axis of the parabola is the x-axis which is also the transverse axis of the parabola. The focus of the parabola is F(a, 0), and the equation of the directrix of this parabola is x + a = 0.

What is the Vertex of the Parabola?

The vertex of the parabola is the point where the parabola cuts through the axis. The vertex of the parabola having the equation y2 = 4ax is (0,0), as it cuts the axis at the origin.

How to Find Equation of a Parabola?

The equation of the parabola can be derived from the basic definition of the parabola. A parabola is the locus of a point that is equidistant from a fixed point called the focus (F), and the fixed-line is called the Directrix (x + a = 0). Let us consider a point P(x, y) on the parabola, and using the formula PF = PM, we can find the equation of the parabola. Here the point 'M' is the foot of the perpendicular from the point P, on the directrix. Hence, the derived standard equation of the parabola is y2 = 4ax.

What is The Eccentricity of Parabola?

The eccentricity of a parabola is equal to 1 (e = 1). The eccentricity of a parabola is the ratio of the distance of the point from the focus to the distance of this point from the directrix of the parabola.

What is the Foci of a Parabola?

The parabola has only one focus. For a standard equation of the parabola y2 = 4ax, the focus of the parabola is F(a, 0). It is a point lying on the x-ais and on the transverse axis of the parabola.

What is the Conjugate Axis of a Parabola?

The line perpendicular to the transverse axis of the parabola and is passing through the vertex of the parabola is called the conjugate axis of the parabola. For a parabola y2 = 4ax, the conjugate axis is the y-axis.

What are The Vertices of a Parabola?

The point on the axis where the parabola cuts through the axis is the vertex of the parabola. The vertex of the parabola for a standard equation of a parabola y2 = 4ax is equal to (0, 0). The parabola cuts the x-axis at the origin.

What is the Standard Equation of a Parabola?

The standard equation of a parabola is used to represent a parabola algebraically in the coordinate plane. The general equation of a parabola can be given as, y = a(x-h)2 + k or x = a(y-k)2 +h, where (h,k) denotes the vertex. The standard equation of a regular parabola is y2 = 4ax.

How to Find Transverse Axis of a Parabola?

The line passing through the vertex and the focus of the parabola is the transverse axis of the parabola. The standard equation of the parabola y2 = 4ax has the x-axis as the axis of the parabola.

The general equation of a parabola is:

y = a(x - h)2 + k (regular)

x = a(y - k)2 + h (sideways)

where,

(h,k) = vertex of the parabola

Where is Parabola Formula Used in Real Life?

Parabolas are used in physics and engineering for the paths of ballistic missiles, the design of automobile headlight reflectors, etc.

How Do you Solve Problems Using Parabola Formula?

To solve problems on parabolas the general equation of the parabola is used, it has the general form y = ax 2 + bx + c (vertex form y = a(x - h) 2 + k) where, (h,k) = vertex of the parabola.

Do all Parabolas Formula Represent a Function?

All parabolas are not necessarily a function. Parabolas that open upwards or downwards are considered functions.

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