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

Sagot :

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:

[tex]y=\dfrac{1}{4p}(x-h)^2+k[/tex]                  ...(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).

[tex](h,k+p)=(6,-4)[/tex]

On comparing both sides, we get

[tex]h=6[/tex]

[tex]k+p=-4[/tex]                            ...(ii)

Directrix at y=-7. So,

[tex]k-p=-7[/tex]                            ...(iii)

Adding (ii) and (iii), we get

[tex]2k=-11[/tex]

[tex]k=\dfrac{-11}{2}[/tex]

[tex]k=-5.5[/tex]

Putting [tex]k=-5.5[/tex] in (ii), we get

[tex]-5.5+p=-4[/tex]

[tex]p=-4+5.5[/tex]

[tex]p=1.5[/tex]

Putting [tex]h=6, k=-5.5,p=1.5[/tex] in (i), we get

[tex]y=\dfrac{1}{4(1.5)}(x-6)^2+(-5.5)[/tex]

[tex]y=\dfrac{1}{6}(x-6)^2-5.5[/tex]

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