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Reviewed CSCA Mathematics question · Easy

A parabola has its vertex at the origin and its focus at (0, 5). What is the equation of its directrix?

  1. y = -5
  2. y = 5
  3. x = -5
  4. x = 5
Show the answer and explanation

Correct answer

A. y = -5

Principle or equation

For a parabola with vertex at (0,0) and focus at (0,p), the directrix is the horizontal line y = -p.

Why this answer is correct

The focus is (0,5), so p = 5. The directrix is y = -5.

Example

For a parabola with focus at (0, -3), the directrix is y = 3.

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