Problem $2.$ Show that the following two equations are the same:

$p_D(r_D,t_D)=-\frac{1}{2}Ei(-\frac{r_D^2}{4t_D})$ and $p(r,t)=p_i-\frac{70\mathrm{.}6qB}{kh}[-E_i(-\frac{948c_{t}r}{kt})]$