Answer
$\iint_{D} f(x,y) dA=\int_{0}^{1} \int_{x}^{1} f(x,y) \ dy \ dx$
Work Step by Step
The domain $D$ in the Type-1 using vertical cross-sections can be expressed as follows: $
D=\left\{ (x, y) | x \leq y \leq 1, \ 0 \leq x \leq 1 \right\}
$
Next, the domain $D$ in the Type-II using horizontal cross-sections can be expressed as follows: $
D=\left\{ (x, y) | 0 \leq x \leq y, \ 0 \leq y \leq 1 \right\}
$
Thus, we have: $\iint_{D} f(x,y) dA=\int_{0}^{1} \int_{x}^{1} f(x,y) \ dy \ dx$
