If [math] denotes the reaction of the body to some stimulus of strength [math], the sensitivity [math] is defined to be the rate of change of the reaction with respect to [math]. A particular example is that when the brightness [math] of a light source is increased, the eye reacts by decreasing the area [math] of the pupil. The experimental formula [math] has been used to model the dependence of [math] on [math] when [math] is measured in sq mm and [math] is measured in the appropriate units of brightness.

Find the sensitivity.