五、(10%) Let \( f(x; \theta) = (1/\theta^2)xe^{-x/\theta} \), where \( x > 0 \) and \( \theta > 0 \). Let \( X_1, X_2, ..., X_n \) denote a random sample of size \( n \) for this distribution. What is the maximum likelihood estimator of \( \theta \). (Please show the details)