3. Let (X1, X2, ..., Xn) be a random sample from a distribution with the probability
density function:
$$f(x) = \begin{cases}
\frac{e^{-x/\theta}}{\theta} & \text{for } x \ge 0 \\
0 & \text{otherwise}
\end{cases}$$
where $\theta > 0$ is a parameter.
(a) Derive the maximum likelihood estimator, $\hat{\theta}$, of $\theta$.
(6 marks)
(b) Check whether $\hat{\theta}$ is an unbiased estimator of $\theta$.
(4 marks)
(c) Calculate the variance of $\hat{\theta}$.
(7 marks)
(d) Explain whether $\hat{\theta}$ is a consistent estimator of $\theta$.
(3 marks)
Hint: You may find the following integral useful:
$$\int_0^\infty x^n e^{-x/\theta} dx = n! \theta^{n+1}$$