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Q 10.

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Found in: Page 643

### Precalculus Enhanced with Graphing Utilities

Book edition 6th
Author(s) Sullivan
Pages 1200 pages
ISBN 9780321795465

# Answer Problem using the figure.If a = 4, then the equation of the directrix is______ .

If a = 4, then the equation of the directrix is

y = -2.

See the step by step solution

## Step 1. Given information.

We have given a = 4 and graph of parabola is,

## Step 2. Concept used.

Directrix of the parabola:

The directrix is perpendicular to the axis of symmetry of a parabola and does not touch the parabola.

Formula for the directrix for upward parabola is: y = k - a.

## Step 3. Explanation.

We have given graph is,

This parabola is upward parabola so directrix of the parabola is,

$y=k-a$.

Where " k " is the y-coordinate of the vertex.

From figure we have vertex = (3, 2).

So equation for the directrix is,

y = 2 - 4 = -2.

## Step 4. Conclusion.

If a = 4, then the equation of the directrix is y = -2.