The brightness of a lamp is L(s) = 1500s^-2 lumens where s is the distance in metres
from the observer to the lamp. An observer is walking at 1 metre per second along a
straight line that comes within 3 metres of the lamp at its closest point. How fast (in
lumens per second) is the brightness changing when the observer has passed the lamp and
is 5 metres away from it?
Thanks in advance