Answer
$D=\{(x,y)\in\mathbb{R}^2| |x+y|\le1\}$
Work Step by Step
The domain of the arcsin function is between -1 and 1, inclusive. Therefore, we have $|x+y|\le1$.
$D=\{(x,y)\in\mathbb{R}^2| |x+y|\le1\}$

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