Biology Arthur and Zettler $^{20}$ created a mathematical model that showed that the percentage mortality $y$ of the red flour beetle was approximated by $y=f(x)=$ $9.1+86.3 e^{-0.64 x},$ where $x$ is the number of weeks after a malathion treatment. Use calculus to show that $f(x)$ is a decreasing function. What happens in the long-term? Sketch a graph. Check using your grapher.