Parabolas |
---|

Let d be a line in the plane and F a fixed point not on d. A parabola is the collection of points in the plane that are equidistant from F and d. The point F is called the focus and the line d is called the directrix. |

The picture illustrates the definition. The point P is a typical point on the parabola so that its distance from the directrix, PQ, is equal to its distance from F, PF. The point marked V is special. It is on the perpendicular line from F to the directix. This line is called the axis of symmetry of the parabola or simply the axis of the parabola and the point V is called the vertex of the parabola. The vertex is the point on the parabola closest to the directrix. |

Finding the equation of a parabola is quite difficult but under certain
cicumstances we may easily find an equation. Let's place the focus and
vertex along the *y* axis with the vertex at the origin.
Suppose the focus is at (0,p). Then the directrix, being perpendicular to
the axis, is a horizontal line and it must be p units away from V. The
directrix then is the line *y*=-p. Consider a point P with
coordinates (x,y) on the parabola and let Q be the point on the directrix
such that the line through PQ is perpendicular to the directrix.
The distance PF is equal to the distance PQ. Rather than use the distance
formula (which involves square roots) we use the square of the distance
formula since it is also true that PF^{2} = PQ^{2}.
We get

x

Although we implied that p was positive in deriving the formula, things work
exactly the same if p were negative. That is if the focus lies on the
negative *y* axis and the directrix lies above the *x* axis the
equation of the parabola is

Another situation in which it is easy to find the equation of a parabola is
when we place the focus on the *x* axis, the vertex at the origin and
the directrix a vertical line parallel to the *y* axis. In this case,
the equation of the parabola comes out to be

Parabolas in standard position |
---|

If we translate a parabola in standard position
both horizontally and vertically we
would like to know the equation of the resulting parabola.
We know that horizontal translations correspond to replacing
*x* with *x*-c and vertical translations correspond
to replacing *y* with *y*-d we obtain the following:

Standard Equation | Shift vertex to | Resulting Equation |

x^{2}=4py |
(h,k) |
(x-h)^{2}=4p(y-k) |

y^{2}=4px |
(h,k) |
(y-k)^{2}=4p(x-h) |

If we take the equation
(*x*-h)^{2}=4p(*y*-k)
and expand it we get
*x*^{2}-2h*x*+h^{2}=4p*y*-4pk
or
*x*^{2}-2h*x*-4p*y*+4pk+h^{2}=0
which is an equation of the form
*x*^{2}+A*x*+B*y*+C=0,
where A, B and C are
constants.
The question we
have is if we are given such an equation can we recognize it as
the equation of a parabola? The answer can be determined by
reversing these steps and this is achieved by completing the square.

Example | Determine if
x^{2}+4x+8y+12=0
is the equation of a parabola. If so, find the coordinates of
the vertex and the focus and the equation of the directrix.
Since only the x terms are quadratic, we only need complete
the square on these terms. We get
x^{2}+4x+8y+12=0
( x+2)^{2}-4+8y+12=0
( x+2)^{2}+8y+8=0
( x+2)^{2}+8(y+1)=0
( x+2)^{2}=-8(y+1)
( x+2)^{2}=4(-2)(y+1)
The equation of the directrix in standard position is y = -(-2). Now we need to translate this 1 unit down. Then the equation of the shifted directrix is y+1 = 2 or y=1.
The picture illustrates the shift of the parabola from standard position to the new position. |

Similarly, if we are given an equation of the form
*y*^{2}+A*y*+B*x*+C=0,
we complete the square on the *y* terms and rewrite in the
form
(*y*-k)^{2}=4p(*x*-h).
From this, we should be able to recognize the coordinates of the
vertex and the focus as well as the equation of the directrix.

In this next exercise set you are given the equation of a parabola in expanded form. On paper, determine the coordinates of the vertex and the focus. When you have your answers, press "Graph" to see the graph of the parabola and the coordinates of the vertex and focus.

Finding the Vertex and Focus of a Parabola |
---|