Question

Write the equation for a parabola with a focus at (6,-4) and a directrix at y= -7

Answer 1

Given:

The focus of the parabola is at (6,-4).

Directrix at y=-7.

To find:

The equation of the parabola.

Solution:

The general equation of a parabola is:

$y=\dfrac{1}{4p}(x-h)^2+k$                  ...(i)

Where, (h,k) is vertex, (h,k+p) is the focus and y=k-p is the directrix.

The focus of the parabola is at (6,-4).

$(h,k+p)=(6,-4)$

On comparing both sides, we get

$h=6$

$k+p=-4$                            ...(ii)

Directrix at y=-7. So,

$k-p=-7$                            ...(iii)

Adding (ii) and (iii), we get

$2k=-11$

$k=\dfrac{-11}{2}$

$k=-5.5$

Putting $k=-5.5$ in (ii), we get

$-5.5+p=-4$

$p=-4+5.5$

$p=1.5$

Putting $h=6, k=-5.5,p=1.5$ in (i), we get

$y=\dfrac{1}{4(1.5)}(x-6)^2+(-5.5)$

$y=\dfrac{1}{6}(x-6)^2-5.5$

Therefore, the equation of the parabola is $y=\dfrac{1}{6}(x-6)^2-5.5$.