## Precalculus (6th Edition) Blitzer

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