Question

4. The reflecting dish of a parabolic microphone has a cross-section in the shape of a parabola. The microphone itself is placed on the focus of the parabola. If the parabola is 60 inches wide and 30 inches deep, how far from the vertex should the microphone be placed?

Answer 1

Assume the parabola is placed on a graph where the x-axis is the top of the dish.
The vertex is then at (0,-30)  The x-intercepts or zeros are at (-30,0) and (30,0)

The equation of such parabola would be:
$y = a(x+30)(x-30)$
Plug in vertex to find value of 'a'
$-30 = a(0+30)(0-30) \\ \\ a = \frac{-30}{(-30)(30)} = \frac{1}{30}$

Now find the focus given that $p = \frac{1}{4a}$
$p = \frac{1}{4(1/30)} = \frac{30}{4} = 7.5$

Answer: the microphone should be placed 7.5 inches from vertex.