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Parabola and its types

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Notes

The equation of a parabola is simplest if the vertex is at the origin and the axis of symmetry is along the x-axis or y-axis. The four possible such orientations of parabola are shown in following fig.

We will derive the equation for the parabola shown above in First  Fig with focus at (a, 0) a > 0; and directricx  x = – a as below: 

Let F be the focus and l the directrix.  Let FM be perpendicular to the directrix and bisect FM at the point O.  Produce MO to X.  By the definition of  parabola, the mid-point O is on the parabola and is called the vertex of the parabola. Take O as origin, OX the x-axis and OY perpendicular to it as the y-axis.  Let the distance from the directrix to the focus be 2a. Then, the coordinates of the focus are (a, 0),  and the equation of the directrix is x + a = 0 as in above fig.
Let P(x, y) be any point on the parabola such that 
PF = PB,                                      ... (1) 
where PB is perpendicular to l. The coordinates of B are (– a, y). 
By the distance formula, we have
PF = `sqrt((x-a)^2+y^2) `and PB = `sqrt(x + a)^2`
Since PF = PB, we have
`sqrt((x-a)^2 + y^2) = sqrt((x+a)^2)`
i.e. `(x – a)^2 + y^2 = (x + a)^2

or x^2 – 2ax + a^2 + y^2 = x^2 + 2ax + a^2`
 or `y^2` = 4ax ( a > 0). 
Hence, any point on the parabola satisfies
`y^2` = 4ax.                    ...(2)
Conversely, let P(x, y) satisfy the equation (2)
PF = `sqrt((x-a)^2+y^2) = sqrt((x-a)^2 + 4ax)`
= `sqrt(x+a)^2` = PB         .....(3)
and  so P(x,y) lies  on the parabola.
Standard equation of Parabola:
`y^2` = 4ax
`y^2` = - 4ax
`x^2` = 4ay
`x^2` = - 4ay

Definition

A parabola is the set of all points in a plane that are equidistant from a fixed line and a fixed point (not on the line) in the plane. 

Notes

The fixed line is called the directrix of the parabola and the fixed point F is called the focus Fig.   (‘Para’ means ‘for’ and ‘bola’ means ‘throwing’, i.e., the shape described when you throw a ball in the air). 

A line through the focus  and perpendicular to the directrix  is called the axis of the parabola. The point of intersection of parabola with the axis is called the vertex of the parabola Fig.

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