Problem 1(Double Integral).
Compute the following double integrals. Be sure to show all steps.
(1) \iint_D 3dA where D is given by $1 \le x \le 3, 2 \le y \le 6$, or equivalently $[1, 3] \times [2, 6]$.
(2) $\int_0^1 \int_0^1 (x^2 + y)dydx$.
(3) $\iint_T (x - 2y)dA$ where T is the triangle with vertices at $(0, 0), (2, 0), and (1, 1)$.