Solution
= Solution
Writing $x=\theta e^{-\beta t}$ gives $\Lambda=N\beta x/(1+x)^2$. Its maximum occurs at $x=1$, or
$$
t_*=\frac{\log\theta}{\beta},
\qquad
\Lambda(t_*)=\frac{N\beta}{4}.
$$
If time is restricted to $t\geq0$ and $\theta<1$, the unconstrained maximizer precedes the initial time, so the maximum on the observed interval is instead at $t=0$.