2. Use polar coordinates to set up and evaluate the double integral $\iint_R f(x, y) dA$.
(a) $f(x, y) = x + y$; $R : x^2 + y^2 \le 4$, $x \ge 0$, $y \ge 0$.
(b) $f(x, y) = \arctan(\frac{y}{x})$, $R : x^2 + y^2 \ge 1$, $x^2 + y^2 \le 4$, $0 \le y \le x$.