Question 11
The directrix is a horizontal line, which means the parabola opens either upward or downward. In this case, it opens downward. This is because all answer choices have a negative leading coefficient. Also, it's because the focus is below the directrix.
For vertically opening parabolas, we use this form
4p(y-k) = (x-h)^2
where (h,k) is the vertex and p is the focal distance, aka the distance from the vertex the focus. To find (h,k), we start at the focus (0,-4) and move directly up until we reach the directrix y = 4. We'll arrive at (0,4). The midpoint of (0,-4) and (0,4) is (0,0) which is the vertex's location. So (h,k) = (0,0).
Note that in moving from (0,-4) to (0,4) is a span of 4 units. So this is the value of p.
Plug h = 0, k = 0, p = 4 into the equation mentioned and solve for y
4p(y-k) = (x-h)^2
4*4(y-0) = (x-0)^2
16y = x^2
y = (1/16)x^2
The only adjustment we need to make is to change the 1/16 to -1/16 so that the parabola opens downward.
Answer: Choice D. y = -(1/16)x^2
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Question 3
The given equation is in the form y = ax^2+bx+c
In this case,
Let's compute the x coordinate of the vertex h
h = -b/(2a)
h = -4/(2*2)
h = -1
This h value is plugged into the original function to find k
f(x) = 2x^2+4x+3
f(-1) = 2(-1)^2+4(-1)+3
f(-1) = 1
We find that h = -1 and k = 1 pair up together. In short, (h,k) = (-1,1) is the vertex.
Answer: Choice B. (-1,1)