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

Find the sensitivity corresponding to .

Sensitivity =