The Pythagorean theorem has a reciprocal variant:

In combination with the inverse-square law, this means that identical lamps placed at A and B will produce the same light intensity at C as a single lamp at D.
The Pythagorean theorem has a reciprocal variant:
In combination with the inverse-square law, this means that identical lamps placed at A and B will produce the same light intensity at C as a single lamp at D.