What is the standard form of the equation of the parabola with a directrix at x=-9 and a focus at (8,4)?

Question

1381 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2017-11-01T08:06:02

The equation of the parabola is #(y-4)^2=17(2x+1)#

Any point #(x,y)# on the parabola is equidistant from the directrix and the focus.

Therefore,

#x-(-9)=sqrt((x-(8))^2+(y-(4))^2)#

#x+9=sqrt((x-8)^2+(y-4)^2)#

Squaring and developing the #(x-8)^2# term and the LHS

#(x+9)^2=(x-8)^2+(y-4)^2#

#x^2+18x+81=x^2-16x+64+(y-4)^2#

#(y-4)^4=34x+17=17(2x+1)#

The equation of the parabola is #(y-4)^2=17(2x+1)#

graph{((y-4)^2-34x-17)((x-8)^2+(y-4)^2-0.05)(y-1000(x+9))=0 [-17.68, 4.83, -9.325, 1.925]}