Solution
= Solution
At <large-deviation speed> $L$, the <exponential distribution> gives the <large deviation principle> on $\mathbb R$ with <good rate function>
$$
\boxed{J(b)=\begin{cases}\lambda b,&b\geq0,\\+\infty,&b<0.\end{cases}}
$$
In particular, $\mathbb P(B/L\geq b)=e^{-\lambda Lb}$ for $b\geq0$. Its finite <sublevel sets> are the <compact sets> $[0,r/\lambda]$.