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Reviewed CSCA Mathematics question · Easy

A parabola has its vertex at the origin and its focus at (0, -4). What is the equation of its directrix?

  1. y = 4
  2. y = -4
  3. x = 4
  4. x = -4
Show the answer and explanation

Correct answer

A. y = 4

Principle or equation

For a parabola with vertex at origin and focus on the y-axis at (0, p), the directrix is y = -p. Here p = -4, so directrix is y = 4.

Why this answer is correct

The focus is (0, -4), so p = -4. The directrix is y = -p = 4.

Example

If focus is (0, 3), directrix is y = -3.

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