Parabola
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 mirrorsymmetrical and usually 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 equations of a parabola, and the properties of a parabola.
1.  What is Parabola? 
2.  Standard Equations of a Parabola 
3.  Parabola Formula 
4.  Graphing Parabola 
5.  Derivation of Parabola Equation 
6.  Properties of Parabola 
7.  FAQs on Parabola 
What is Parabola?
A parabola refers to an equation of a curve, such that each 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. Thus, a parabola is mathematically defined as follows:
"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(xh)^{2} + k or x = a(yk)^{2} +h, where (h,k) denotes the vertex. The standard equation of a regular parabola is y^{2} = 4ax.
Some of the important terms below are helpful to understand the features and parts of a parabola y^{2} = 4ax.
 Focus: The point (a, 0) is the focus of the parabola
 Directrix: The line drawn parallel to the yaxis 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.
Here are the formulas to find the equation of the axis, directrix, vertex, focus, and length of the latus rectum of different types of parabolas.
Standard Equations of a Parabola
There are four standard equations of a parabola.
 y^{2} = 4ax
 y^{2} = 4ax
 x^{2} = 4ay
 x^{2} = 4ay
The below image presents the four standard equations and forms of the 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 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 y^{2}, then the axis of symmetry is along the xaxis and if the equation has the term with x^{2}, then the axis of symmetry is along the yaxis.
 When the axis of symmetry is along the xaxis, 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 yaxis, 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.
Parabola Formulas  

Equation  y = a(x  h)^{2} + k  x = a(y  k)^{2} +h 
Axis of Symmetry  x = h  y = k 
Vertex  (h, k)  (h, k) 
Focus  (h, k + (1/4a))  (h + (1/4a), k) 
Directrix  y = k  1/4a  x = h  1/4a 
Direction of Opening  Up (a > 0) or Down (a < 0)  Right (a > 0) or Left (a < 0) 
Length of Latus Rectum  1/a  1/a 
Does it Have Max or Min?  Max if a < 0 Min if a > 0 
Not Applicable 
Graphing Parabola
Consider an equation y = 3x^{2}  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)
Length of latus rectum = 1/a = 1/3
Focus: (h, k + 1/4a) = (1,25/12)
Axis of symmetry is x =1
Directrix: y = k1/4a
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} + y^{2} = (x + a)^{2}
x^{2} + a^{2}  2ax + y^{2} = x^{2} + a^{2} + 2ax
y^{2}  2ax = 2ax
y^{2 }= 4ax
Now we have successfully derived the standard equation of a parabola.
Similarly, we can derive the equations of the other types of parabolas as:
 (b): y^{2} = – 4ax,
 (c): x^{2} = 4ay,
 (d): x^{2} = – 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 y^{2} = 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 y^{2} = 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 y^{2} =4ax, the equation of the polar is \(yy_1 = 2x(x + x_1)\).
Parametric Coordinates: The parametric coordinates of the equation of a parabola y^{2} = 4ax are (at^{2}, 2at). The parametric coordinates represent all the points on the parabola.
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Parabola Examples

Example 1: The equation of parabola is y^{2 }= 24x. Find the length of the latus rectum, focus, and vertex.
Solution:
To find: Length of latus rectum, focus and vertex of the parabola
Given: Equation of a parabola: y^{2 }= 24x
Therefore, 4a = 24
a = 24/4 = 6
Now, parabola formula for latus rectum is:
Length of latus rectum = 4a
= 4(6) = 24
Now, focus= (a,0) = (6,0)
Now, Vertex = (0,0)
Answer: Length of latus rectum = 24, focus = (6,0), vertex = (0,0)

Example 2: The equation of a parabola is 2(y3)^{2} + 24 = x. Find the length of the latus rectum, focus, and vertex.
Solution:
To find: the length of the latus rectum, focus and vertex of a parabolaGiven: equation of a parabola: 2(y3)^{2 }+ 24 = x
On comparing it with the general parabola equation, x = a(yk)^{2 }+ h, we get
a = 2
Length of latus rectum = 1/a = 1/2
Vertex, (h, k) = (24, 3)
Focus = (h + 1/4a, k) = (24 + 1/8, 3) = (193/8, 3)Answer: Length of latus rectum = 1/2, focus = (193/8, 3), Vertex = (24,3)

Example 3. Which equation represents a parabola that has a focus of (0, 0) and a directrix of y = 4?
Solution:
Given that, Focus = (0, 0) and directrix y = 4
Let us suppose that there is a point (x, y) on the parabola.
Its distance from the focus point (0, 0) is √((x − 0)^{2} + (y  0)^{2} )
Its distance from directrix y = 4 is y  4
By the definition of parabola, these two distances are the same.
√[(x − 0)^{2} + (y  0)^{2}] = y  4
Squaring on both sides.
(x − 0)^{2} + (y  0)^{2} = (y  4)^{2}
x^{2} + y^{2} = y^{2}  8y + 16
x^{2} + 8y  16 = 0
Answer: Hence, the equation of parabola with a focus at (0, 0) and a directrix of y = 4 is x^{2} + 8y  16 = 0.
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 y^{2} = 4ax. The axis of this parabola is the xaxis 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.
How to Graph a Parabola?
For graphing parabola:
 Step 1: Find the vertex of parabola
 Step 2: Find some other points on the parabola by taking
 random values for x if its a up/down open parabola
 random values for y if its a left/down open parabola
 Step 3: Plot the vertex and the points found in Step 1 and Step 2 and join them by a smooth curve.
What is the Vertex of the Parabola?
The vertex of the parabola is the point where the parabola changes its direction. The vertex of the parabola having the equation y^{2} = 4ax is (0,0), and it has either maximum or minimum at this point.
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 fixedline 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 form of parabola is y^{2} = 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 y^{2} = 4ax, the focus of the parabola is F(a, 0). It is a point lying on the xais 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 passing through the vertex of the parabola is called the conjugate axis of the parabola. For a parabola y^{2} = 4ax, the conjugate axis is the yaxis.
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(xh)^{2} + k or x = a(yk)^{2} +h, where (h,k) denotes the vertex. The standard form of parabola is y^{2} = 4ax or x^{2} = 4ay.
How to Find the 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 parabola standard form y^{2} = 4ax has the xaxis as the axis 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 parabolic functions. The parabolas that open to left or right side fail vertical line test and hence are NOT functions.
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