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Derive the equation of the parabola with a focus at (-2,4) and a directrix of y=6 . Put the equation in standard form

User Thao Ngo
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1 Answer

5 votes

Answer:


y = - (1)/(4) {(x + 2)}^(2) + 5

Explanation:

The vertex of this parabola is the midpoint of the focus (-2,4) and where the directrix intersects the axis of symmetry of the parabola (-2,6)

This parabola must open downwards due to the position of the directrix and has equation of the form:


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

where (h,k) is the vertex.

This implies that:


h = - 2

and


k = (4 + 6)/(2) = 5

The value of p is the distance from the vertex to the focus:


p = |6 - 5| = 1

We substitute all the values into the formula to get:


(x - - 2)^(2) = - 4(1){(y - 5)}


{(x + 2)}^(2) = - 4(y - 5)

Or


y = - (1)/(4) {(x - 5)}^(2) + 5

User Ohrstrom
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