Suppose that $X_{1}, X_{2}, \dots, X_{n}$ are independent random variables each drawn from a population having density function $f_{X}(x)=\begin{cases} \frac{1}{\theta} e^{-(x-\mu)/\theta} & \text{if } x \ge \mu \\ 0 & x < \mu \end{cases}$ where $\theta > 0$ and $\mu \in R^{+}$, then maximum likelihood estimate of $(\theta, \mu)$, when both $\theta, \mu$ are unknown is