Parabola Formula

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 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 (x1,y1)

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)

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 (x1,y1) is yy1=2a(x+x1)

.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 (x1,y1) and having a slope of m = -y1/2a, the equation of the normal is (y−y1)=−y12a(x−x1)

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 (x1,y1) outside the parabola, the equation of the chord of contact is yy1=2x(x+x1)

.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 (x1,y1), for a parabola y² =4ax, the equation of the polar is yy1=2x(x+x1)

.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.

Parabola Examples

Definition and Equation of Parabola

A parabola is a section of the right cone that is parallel to one side (a producing line) of the conic figure. Much the same as the circle, the parabola is also a quadratic relation, but different from the circle, either ‘A’ will be squared or ‘B’ will be squared, but never both. That said, a parabola is a set of all points M(A, B) in a plane in a way that the distance from M to a definite point F known as the focus is equivalent to the distance from M to a definite line known as the directrix as shown below in the graph.

Parabolic Equation

The general equation of a parabola is y = x² in which x-squared is a parabola.  Work up its side it becomes y² = x or mathematically expressed as y = √x

The Formula for Equation of a Parabola

Taken as known the focus (h, k) and the directrix y = mx+b, parabola equation is y−mx–by−mx–by – mx – b² / m²+1m²+1m² +1 = (x – h)² + (y – k)² .

Spot the Parabola at a Stroke

The shape of the parabola is what you see when you buy an ice cream cone and snip it off parallel to the side of the cone.

Usefulness of Parabola

Think of simply an arc and you will know what a parabola is. Now you will learn about parabolas and their standard form equations since these equations are quite frequently applicable in real life.

FAQs (Frequently Asked Questions)

1. What are the properties of a parabola?

The properties of a parabola are given below:

• Tangent: It is a line touching the parabola.
• Normal: The normal is a line drawn perpendicular to the tangent that passes through the point of contact and the focus of the parabola.
• Chord of contact: A chord of contact is a chord drawn to join the point of contact of the tangents drawn from an external point to the parabola. 
• Pole and polar: Polar is the point lying outside the parabola, the locus of the points of intersection of the tangents, drawn at the ends of the chords. The referred point is called the pole. 
• Parametric coordinates: The parametric coordinates are the points on the parabola.

2. What makes a parabola to magnify satellite signals?

Apart from how resourceful parabolas are, there is another remarkable explanation of a parabola. It is this explanatory that enables the parabola to amplify clusters of tiny signals to turn into a one big clear signal. But aren’t you curious to know what exactly this is? It is actually the point on a parabola that’s called the focus. This point is positioned at the interior of the curve of your parabola. It is that point on the parabola where the distance from any point on the parabola to this focus point equals the distance from the point to a straight line at the exterior of the parabola.

3. Ever wondered why the focus in parabola called the focus?

A focus is termed focus typically because of the manner they are framed and also because of the characteristic of the focus, that parabolas qualify to get hold of signals and then reflect all of them to a single focus point itself just so they become one big clear signal. Yes, parabolas are competent to concentrate signals to a single point.

4. What information do we need to write the standard form of the parabola equation?

We only require a piece of information about our parabola i.e. the position of the vertex and the position of our focus.

Parabola

Measurements for a Parabolic Dish

If you want to build a parabolic dish where the focus is 200 mm above the surface, what measurements do you need?

To make it easy to build, let’s have it pointing upwards, and so we choose the x² = 4ay equation.

And we want “a” to be 200, so the equation becomes:

x² = 4ay = 4 × 200 × y = 800y

Rearranging so we can calculate heights:

y = x²/800

And here are some height measurements as you run along:

Example 3

Put the equation x = 5 y ² – 30 y + 11 into the form

x = a( y – k) ² + h

Determine the direction of opening, vertex, focus, directrix, and axis of symmetry.

x = 5 y ² – 30 y + 11

Factor out the coefficient of y ² from the terms involving y so that you can complete the square.

x = 5( y ² – 6 y) + 11

Completing the square within the parentheses adds 5(9) = 45 to the right side. Add this amount to the left side to keep the equation balanced.

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