9.1.38 Double integrals

Problem: In the double integral, make the transition to the iterated integral: \[ \iint_{G} f(x) d x d y \text {, } \] a) \( G:\left\{\begin{array}{c}x=1 \\ y=x^{2}, y=2 x(x \leq 1)\end{array}\right. \), b) \( G:\left\{\begin{array}{c}x=0, y=0 \\ x^{2}+y^{2}=a^{2}(x \geq 0)\end{array} \quad(a \geq 0)\right. \).