(40%) Let $g(x)$ be an unbiased estimator for the estimation of $\theta$ with the
observations of the random variables $X_1$, $X_2$, ..., $X_N$ under the joint PDF
$f(X_1 = x_1, X_2 = x_2, ..., X_N = x_N; \theta)$ denoted by $f(x; \theta)$.
(a). (10%) Using the unbiased property, show that $\int_{-\infty}^{\infty} (g(x) - \theta) \frac{\partial f(x; \theta)}{\partial \theta} dx = 1$.
Hint: Unbiased property: $E\{g(x) - \theta\} = \int_{-\infty}^{\infty} (g(x) - \theta) f(x; \theta) dx = 0$.