2. Let $X_1$ and $X_2$ be jointly continuous random variables with probability density function:
$f_{X_1, X_2}(x_1, x_2)$. Let $Y_1 = X_1 + X_2$ and $Y_2 = X_1 - X_2$,
a. Find the joint density function $f_{Y_1, Y_2}(y_1, y_2)$,
b. If $X_1$ and $X_2$ are independent and uniform in the interval $[0, 1]$ random variables,
find the joint pdfs of $f_{X_1, X_2}(x_1, x_2)$, and $f_{Y_1, Y_2}(y_1, y_2)$
