Answer
The light bulb must be placed at 4 inches above the vertex.
Work Step by Step
As per the given statement , we have $y=1$
and Diameter = 8 inches. This implies that Radius: $2x=8 \implies x=4$
Now, consider $x^2=4py$
This implies that $(4)^2=(4p)(1) \implies p=4$
Hence, the light bulb must be placed at 4 inches above the vertex.
