Answer:
-1/32 × x
Step-by-step explanation:
let's define and identify the correct markers of the correct parabola.
so, we have the focus point F (0, -8).
we have the directrix y = 8.
that means the the vertex point (the middle between the focus and the directrix) is (0, 0).
a parabola is turning away from the directrix. the vertex here is below the directrix, so the parabola is opening downwards and has therefore a negative factor of x.
so, we can rule out the 2 positive answer options.
to decide between the other 2, let's use a value of x and see, if the point is equidistant to the focus and the directrix.
e.g. x = 1
so,
y = -1/8 × x²
gives us y = -1/8 for x = 1
that means the point is (1, -1/8).
the distance to the focus (0, -8) is
(1 - 0)² + (-1/8 - - 8)² = 1 + (7 7/8)² = 1 + (63/8)² =
= 64/64 + 3969/64 = 4033/64
the distance to the directrix is the distance to the point (1, 8) as the point on the directrix must have the same x value.
so, the distance (1, -1/8) to (1, 8) is
(1 - 1)² + (-1/8 - 8)² = 0 + (-65/8)² = 4225/64
that is not equal to the distance to the focus.
so, let's try
y = -1/32 × x
the point for x = 1 is (1, -1/32)
distance to focus
(1, -1/32) to (0, -8) = (1-0)² + (-1/32 - -8)² = 1 + (255/32)² =
= 1024/1024 + 65025/1024 = 66049/1024
distance to directrix
(1, -1/32) to (1, 8) = (1-1)² + (-257/32)² = 66049/1024
which is the same distance as to the focus.
so, y = -1/32 × x is the right answer.
FYI - for an actual distance I would have to create the square roots of the numbers used here.
but since we only needed to see, if distances are equal or not, it was sufficient to check if the squares of the distances are equal or not.
