$($a$)$ Use MuPAD to show that

$\underset{x}{lim}{e}^{-ax}=\{\begin{array}{ccc}0& \text{f}\text{o}\text{r}& a>0\\ 1& \text{f}\text{o}\text{r}& a=0\\ & \text{f}\text{o}\text{r}& a<0\end{array}$

Hint: Use the function assume to distinguish the cases.