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 x of a light source is increased, the eye reacts by decreasing the area R (square millimeters) of the pupil. A formula used to model the dependence of R on x is: R(x) = (40+ 24x^(2/5)) / (1 + 4x^(2/5)). The sensitivity can be found by the derivative S = dR/dx = R'(x). This derivative can be found by... dividing the derivative of the numerator by the derivative of the denominator with the use of the Power Rule, such as R'(x) = [24 (2/5) x^(-3/5)] / [4 (2/5) x^(-3/5)], and then simplifying "cancelling" the x^(2/5) in the original function, and then reducing R(x) = (40+24)/(1+4) to 64/5, which is a constant having derivative R' (x) = 0 dividing the derivative of the denominator by the derivative of the numerator with the use of the Power Rule, such as R'(x) = [4 (2/5) x ^(-3/5)] / [24 (2/5) x^(-3/5)], and then simplifying applying the Quotient Rule on the overall problem and the Power Rule in individual derivatives, such as R' (x) = [(1 + 4x^(2/5))*(24 (2/5)x^(-3/5)) -(40+24x^(2/5))*(4 (2/5)x^(-3/5))] / (1 + 4x^(2/5)) ^ 2, and then simplifying