By using the properties of definite integrals,evaluate the integral $\int_{0}^{1} x(1-x)^{n} d x$.

Let $I = \int_{0}^{1} x(1-x)^{n} d x$.
Using the property $\int_{0}^{a} f(x) d x = \int_{0}^{a} f(a-x) d x$,we get:
$I = \int_{0}^{1} (1-x)(1-(1-x))^{n} d x$
$I = \int_{0}^{1} (1-x)(x)^{n} d x$
$I = \int_{0}^{1} (x^{n} - x^{n+1}) d x$
Integrating term by term:
$I = \left[ \frac{x^{n+1}}{n+1} - \frac{x^{n+2}}{n+2} \right]_{0}^{1}$
Evaluating at the limits:
$I = \left( \frac{1^{n+1}}{n+1} - \frac{1^{n+2}}{n+2} \right) - (0 - 0)$
$I = \frac{1}{n+1} - \frac{1}{n+2}$
$I = \frac{(n+2) - (n+1)}{(n+1)(n+2)}$
$I = \frac{1}{(n+1)(n+2)}$