Question

Fabio is designing a flashlight that uses a parabolic reflecting mirror and a light source. The shape of the mirror can be modeled by (x-2)^2=10(y-3), where x and y are measured in inches. Where is the focus of the flashlight? A. (–2, –0.5) B. (2, –5.5) C. (2, 5.5) D. (5.5, 2)

Answer 1

Answer:

(2, 11/2)

Step-by-step explanation:

This is a vertical parabola; we know that because x is squared here, while y is not.  The standard equation of a vertical parabola with vertex (h,k) is

4p(y-k) = (x-h)^2, where p is the distance between the vertex and the focus.  Comparing

4p(y-k) = (x-h)^2 to

10(y-3)  = (x-2)^2, we see that 4p = 10.  Therefore, p = 10/4 = 5/2, which is the vertical distance between the focus and the vertex.

Since the coordinates of the vertex are easily read from the given equation

(x-2)^2=10(y-3):  (h,k) = (2, 3)

all we need to do is to add p (5/2) to the y-coordinate (3);

The focus is at (2, 3 + 5/2), or (2, 11/2).