SOLUTION: A flashlight is shaped like a paraboloid, so that if its light bulb is placed at the focus, the light rays from the bulb will then bounce of the surface in a focused direction th

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Question 1180349: A flashlight is shaped like a paraboloid, so that if its light bulb is placed at the focus, the light rays from the bulb will then bounce of the surface in a
focused direction that is parallel to the axis. If the paraboloid has a depth of
1.8 in and the diameter on its surface is 6 in, how far should the light source
be placed from the vertex?​

Answer by greenestamps(13200) About Me  (Show Source):
You can put this solution on YOUR website!


Consider a cross section of the paraboloid as a parabola with vertex at the origin. Then the equation of the parabola is

y=%281%2F%284p%29%29x%5E2

where p is the distance from the vertex to the focus.

With a depth of 1.8 inches and a diameter of 6 inches, a point on the parabola is (3,1.8). Use that point in the equation of the parabola to determine p.

1.8+=+%281%2F%284p%29%29%283%5E2%29
1.8%2F9+=+1%2F5+=+1%2F%284p%29
4p=5
p=5%2F4=1.25

ANSWER: 1.25 inches