4. Let \( X \) have the probability density function given by: \[ f_X(x) = \begin{cases} \frac{x+1}{2}, & \text{if } -1 \le x \le 1 \\ 0, & \text{elsewhere} \end{cases} \] Find the density function for \( U = X^2 \) and the mean value \( E[U] = ? \) (10 分)