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 $R$ of the pupil. The experimental formula $\displaystyle R=\frac{40+24x^{.4}}{1+4x^{.4}}$ has been used to model the dependence of $R$ on $x$ when $R$ is measured in sq mm and $x$ is measured in the appropriate units of brightness.

Find the sensitivity.

$R'(x)=$