Question

A parabola can be represented by the equation x2 = –20y.

What are the coordinates of the focus of the parabola?

(–5,0)
(5,0)
(0,5)
(0,–5)

Answer 1

(0,-5)

Explanation:

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Answer 2

(0, - 5)

Explanation:

For a quadratic equation:

y = a*x^2 + b*x + c

if the vertex is at:

(h, k)

Then the focus is at:

(h, k + 1/(4*a))

Here we have the parabola:

x^2 = -20*y

if we isolate y, we get:

y = (-1/20)*x^2

First, we need to find the vertex.

remember that for a general quadratic equation

a*x^2 + b*x + c = 0

the x-value of the vertex is at:

x = -b/2*a

in this case we have b = 0, then our vertex is at x = 0.

If we evaluate the function at x = 0 we get:

y = (-1/20)*0^2 = 0

Then the vertex is at (0, 0)

Thus the focus will be at:

(0, 0 + 1/(4*a))

and we have

a = -1/20

then:

1/(4*a) = -20/4 = 5

Then the focus is:

(0, 0 -5) = (0, - 5)