If R denotes the reaction of the body to some stimulus of strength x, the sensitivity S is dened to be the rate of change of the reaction with...

If R denotes the reaction of the body to some stimulus of strength x, the sensitivity S is defined to be the rate of change of the reaction with respect to x. A particular example is that when the brightness of x of a light source is increased, the eye reacts by decreasing the area R of the pupil. The experimental formula R = 40 + 24x 0.4 1 + 4x0.4 has been used to model the dependence of R on x when R is measured in square millimeters and x is measured in appropriate brightness. Find the sensitivity.